Papers - SUSA Hajime
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Amplification and Saturation of Turbulent Magnetic Fields in Collapsing Primordial Gas Clouds International coauthorship
Higashi, Sho, Susa, Hajime, Federrath, Christoph, Chiaki, Gen
The Astrophysical Journal 2024.2
Recent numerical studies suggest that magnetic fields play an important role in primordial star formation in the early Universe. However, the detailed evolution of the magnetic field in the collapse phase still has uncertainties because of the complicated physics associated with turbulence in a collapsing magnetized system. Here, we perform a suite of numerical MHD simulations that follow the collapse of magnetized, turbulent primordial gas clouds to investigate the evolution of the magnetic field associated with the turbulence, assuming a polytropic equation of state with exponent γ <SUB>eff</SUB> and with various numerical resolutions. In addition, we generalize the analytic theory of magnetic field growth/saturation so that it can deal with various exponents γ <SUB>eff</SUB> and turbulence energy spectra. We find that the numerical results are well reproduced by the theory for various γ <SUB>eff</SUB> through the collapse phase during the formation of the first stars. The magnetic field is eventually amplified by a factor of 10<SUP>12</SUP>–10<SUP>15</SUP> due to kinematic and nonlinear turbulent dynamo effects and reaches 3%–100% of the equipartition level, depending on γ <SUB>eff</SUB>. We also find that the transition between the kinematic and nonlinear stages can be analytically estimated. These results indicate that the strong magnetic field accompanied by supersonic turbulence is a general property and suggest that it can play a crucial role in the formation of the first stars....
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Merger Conditions of Population III Protostar Binaries
Takanobu Kirihara, Hajime Susa, Takashi Hosokawa, Tomoya Kinugawa
The Astrophysical Journal 950 ( 2 ) 188 - 188 2023.6
Publisher：American Astronomical Society
Abstract
Massive close binary stars with extremely small separations have been observed, and they are possible progenitors of gravitational-wave sources. The evolution of massive binaries in the protostellar accretion stage is key to understanding their formation process. We, therefore, investigate how close the protostars, consisting of a high-density core and a vast low-density envelope, can approach each other but not coalesce. To investigate the coalescence conditions, we conduct smoothed particle hydrodynamics simulations following the evolution of equal-mass binaries with different initial separations. Since Population (Pop) I and III protostars have similar interior structures, we adopt a specific Pop III model with the mass and radius of 7.75 M<sub>⊙</sub> and 61.1 R<sub>⊙</sub> obtained by the stellar evolution calculations. Our results show that the binary separation decreases due to the transport of the orbital angular momentum to spin angular momentum. If the initial separation is less than about 80% of the sum of the protostellar radius, the binary coalesces in a time shorter than the tidal lock timescale. The mass loss up to the merging is ≲3%. After coalescence, the star rotates rapidly, and its interior structure is independent of the initial separation. We conclude that there must be some orbital shrinking mechanism after the protostars contract to enter the zero-age main-sequence stage.Other Link： https://iopscience.iop.org/article/10.3847/1538-4357/acd1e0/pdf
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Saturation Level of Turbulence in Collapsing Gas Clouds Reviewed
Sho Higashi, Hajime Susa, Gen Chiaki
The Astrophysical Journal 940 ( 1 ) 1 - 10 2022.11
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Amplification of Turbulence in Contracting Prestellar Cores in Primordial Minihalos Reviewed
Sho Higashi, Hajime Susa, Gen Chiaki
The Astrophysical Journal 915 ( 2 ) 107 - 107 2021.7
Publisher：American Astronomical Society
We investigate the amplification of turbulence through gravitational
contraction of the primordial gas in minihalos. We perform numerical
simulations to follow the cloud collapse, assuming polytropic equations of
state for different initial turbulent Mach numbers and resolutions. We find
that the turbulent velocity is amplified solely by gravitational contraction,
and eventually becomes comparable to the sound speed, even for small initial
turbulent Mach numbers (${\cal M}_0 \gtrsim 0.05$). We derive an analytic
formula for the amplification of turbulent velocity in a collapsing cloud, and
find that our numerical results are consistent with the formula. These results
suggest that the turbulence can play an important role in collapsing clouds for
general cases.Other Link： https://iopscience.iop.org/article/10.3847/1538-4357/ac01c7/pdf
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Merge or Survive: Number of Population III Stars per Minihalo Reviewed
Hajime Susa
The Astrophysical Journal 877 ( 2 ) 99 2019.6
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Metal-poor star formation triggered by the feedback effects from Pop III stars Reviewed
Gen Chiaki, Hajime Susa, Shingo Hirano
Monthly Notices of the Royal Astronomical Society 475 ( 4 ) 4378 - 4395 2018.2
Joint Work
DOI： 10.1093/mnras/sty040
Other Link： https://academic.oup.com/mnras/article-abstract/475/4/4378/4862586?redirectedFrom=PDF
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Where are the Low-mass Population III Stars? Reviewed
Ishiyama, Tomoaki; Sudo, Kae; Yokoi, Shingo; Hasegawa, Kenji; Tominaga, Nozomu; Susa, Hajime
The Astrophysical Journal 826 ( 1 ) id.9 2016.7
Joint Work
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The Mass Spectrum of the First Stars Reviewed
Susa, Hajime; Hasegawa, Kenji; Tominaga, Nozomu
The Astrophysical Journal 792 ( 1 ) 1d.32 2014.9
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Generation of a Seed Magnetic Field around First Stars: The Biermann Battery Effect Reviewed
Kentaro Doi and Hajime Susa
The Astrophysical Journal 741 ( 2 ) 93 2011.11
Joint Work
DOI： 10.1088/0004-637X/741/2/93
Other Link： http://iopscience.iop.org/0004-637X/741/2/93/
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Smoothed Particle Hydrodynamics coupled with Radiation Transfer
須佐元
Publications of the Astronomical Sciety of Japan 58 ( 445 ) 2006.11
Single Work
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Formation of dwarf galaxies during the cosmic reionization
Susa H, Umemura M.
Astrophysical Journal 600 ( 1 I ) 1 - 16 2004.1
Joint Work
We reanalyze the photoevaporation problem of subgalactic objects irradiated by ultraviolet background (UVB) radiation in a reionized universe. For the purpose, we perform three-dimensional radiation smoothed particle hydrodynamics (RSPH) calculations, in which the radiative transfer is solved by a direct method and the nonequilibrium chemistry of primordial gas including H2 molecules is also incorporated. Attention is concentrated on radiative transfer effects on the UVB for the formation of subgalactic objects with Tvir<~104 K. We consider a reionization model with zreion~7 and also the earlier reionization model (zreion~17) inferred by the Wilkinson Microwave Anisotropy Probe (WMAP). We find that star formation is suppressed appreciably by UVB, but baryons at high-density peaks are self-shielded even during the reionization, forming some amount of stars eventually. In that sense, the photoevaporation for subgalactic systems is not as perfect as argued by one-dimensional spherical calculations. The final stellar fraction depends on the collapse epoch and the mass of the system, almost regardless of the reionization epoch. For instance, a few tenths of the formed stars are born after the cosmic reionization in the zreion~7 model, while more than 90% of the stars are born after the reionization in the WMAP reionization model. Thus, the effects of UVB feedback on the substructure problem with a cold dark matter (CDM) scenario should be evaluated with careful treatment of the radiative transfer. The star clusters formed at high-density peaks coalesce with each other in a dissipationless fashion in a dark matter potential, as a resultant forming a spheroidal system. As a result, these low-mass galaxies have large mass-to-light ratios, such as observed in dwarf spheroidal galaxies in the Local Group....
DOI： 10.1086/379784
Other Link： http://orcid.org/0000-0002-3380-5302
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The Thermal Evolution of the Postshock Layer in Pregalactic Clouds
Susa H, Uehara H, Nishi R, Yamada M.
Progress of Theoretical Physics 100 ( 1 ) 63 - 77 1998.7
Joint Work
We re-examine the thermal evolution of the postshock layer in primordial gas clouds. Comparing the time scales, we find that the evolutionary paths of postshock regions in primordial gas clouds can be basically understood in terms of the diagram drawn in the ionization degree vs temperature plane. The results obtained from the diagram are independent of the density in the case that we do not include photodissociation and photoionization. We also argue that the diagram is not only relevant to the case of the steady postshock flow, but also to the isochorically cooling gas....
DOI： 10.1143/PTP.100.63
Other Link： http://orcid.org/0000-0002-3380-5302
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Tanikawa, Ataru, Yoshida, Takashi, Kinugawa, Tomoya, Trani, Alessandro A., Hosokawa, Takashi, Susa, Hajime, Omukai, Kazuyuki
The Astrophysical Journal 926 ( 1 ) 83 - 83 2022.2
Publisher：American Astronomical Society
<title>Abstract</title>
We investigate the formation of merging binary black holes (BHs) through isolated binary evolution, performing binary population synthesis calculations covering an unprecedentedly wide metallicity range of Population (Pop) I, II, III, and extremely metal-poor (EMP) binary stars. We find that the predicted merger rate density and primary BH mass (<italic>m</italic>
<sub>1</sub>) distribution are consistent with the gravitational wave (GW) observations. Notably, Population III and EMP (<10<sup>−2</sup>
<italic>Z</italic>
<sub>⊙</sub>) binary stars yield most of the pair instability (PI) mass gap events with <italic>m</italic>
<sub>1</sub> = 65–130 <italic>M</italic>
<sub>⊙</sub>. Population III binary stars contribute more to the PI mass gap events with increasing redshift, and all the PI mass gap events have the Population III origin at redshifts ≳8. Our result can be assessed by future GW observations in the following two points. First, there are no binary BHs with <italic>m</italic>
<sub>1</sub> = 100–130 <italic>M</italic>
<sub>⊙</sub> in our result, and thus the <italic>m</italic>
<sub>1</sub> distribution should suddenly drop in the range of <italic>m</italic>
<sub>1</sub> = 100–130 <italic>M</italic>
<sub>⊙</sub>. Second, the PI mass gap event rate should increase toward higher redshift up to ∼11, since those events mainly originate from the Population III binary stars. We find that the following three assumptions are needed to reproduce the current GW observations: a top-heavy stellar initial mass function and the presence of close binary stars for Population III and EMP binary stars, and inefficient convective overshoot in the main-sequence phase of stellar evolution. Without any of the above, the number of PI mass gap events becomes too low to reproduce current GW observations.Other Link： https://iopscience.iop.org/article/10.3847/1538-4357/ac4247/pdf
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Gravitational Wave Physics and Astronomy in the nascent era Reviewed
Arimoto et al.
Progress of Theoretical and Experimental Physics 2021.4
DOI： 10.1093/ptep/ptab042
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Ionization degree and magnetic diffusivity in star-forming clouds with different metallicities
Nakauchi, Daisuke, Omukai, Kazuyuki, Susa, Hajime
Monthly Notices of the Royal Astronomical Society 502 ( 3 ) 3394 - 3416 2021.4
Publisher：OXFORD UNIV PRESS
Magnetic fields play such essential roles in star formation as transporting
angular momentum and driving outflows from a star-forming cloud, thereby
controlling the formation efficiency of a circumstellar disc and also multiple
stellar systems. The coupling of magnetic fields to the gas depends on its
ionization degree. We calculate the temperature evolution and ionization degree
of a cloud for various metallicities of Z/Zsun = 1e-6, 1e-5, 1e-4, 1e-3, 1e-2,
1e-1, and 1. We update the chemical network by reversing all the gas-phase
processes and by considering grain-surface chemistry, including grain
evaporation, thermal ionization of alkali metals, and thermionic emission from
grains. The ionization degree at nH ~ 1e15-1e19 /cm^3 becomes up to eight
orders of magnitude higher than that obtained in the previous model, owing to
the thermionic emission and thermal ionization of K and Na, which have been
neglected so far. Although magnetic fields dissipate owing to ambipolar
diffusion or Ohmic loss at nH < 1e15 /cm^3, the fields recover strong coupling
to the gas at nH ~ 1e15 /cm^3, which is lower by a few orders of magnitude
compared to the previous work. We develop a reduced chemical network by
choosing processes relevant to major coolants and charged species. The reduced
network consists of 104 (161) reactions among 28 (38) species in the absence
(presence, respectively) of ionization sources. The reduced model includes H2
and HD formation on grain surfaces as well as the depletion of O, C, OH, CO,
and H2O on grain surfaces.Other Link： http://arxiv.org/pdf/2101.10850v1
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Tanikawa, Ataru, Susa, Hajime, Yoshida, Takashi, Trani, Alessandro A., Kinugawa, Tomoya
The Astrophysical Journal 910 ( 1 ) 30 - 30 2021.3
Publisher：American Astronomical Society
We present the merger rate density of Population (Pop.) III binary black
holes (BHs) by means of a widely-used binary population synthesis code BSE with
extensions to very massive and extreme metal-poor stars. We consider not only
low-mass BHs (lBHs: $5-50 M_\odot$) but also high-mass BHs (hBHs: $130-200
M_\odot$), where lBHs and hBHs are below and above the pair-instability mass
gap ($50-130 M_\odot$), respectively. Pop. III BH-BHs can be categorized into
three subpopulations: BH-BHs without hBHs (hBH0s: $m_{\rm tot} \lesssim 100
M_\odot$), with one hBH (hBH1s: $m_{\rm tot} \sim 130-260 M_\odot$), and with
two hBHs (hBH2s: $m_{\rm tot} \sim 270-400 M_\odot$), where $m_{\rm tot}$ is
the total mass of a BH-BH. Their merger rate densities at the current universe
are $\sim 0.1$ yr$^{-1}$ Gpc$^{-3}$ for hBH0s, and $\sim 0.01$ yr$^{-1}$
Gpc$^{-3}$ for the sum of hBH1s and hBH2s, provided that the mass density of
Pop. III stars is $\sim 10^{13} M_\odot$ Gpc$^{-3}$. These rates are modestly
insensitive to initial conditions and single star models. The hBH1 and hBH2
mergers can dominate BH-BHs with hBHs discovered in near future. They have low
effective spins $\lesssim 0.2$ in the current universe. The number ratio of the
hBH2s to the hBH1s is high, $\gtrsim 0.1$. We also find BHs in the mass gap (up
to $\sim 85 M_\odot$) merge. These merger rates can be reduced to nearly zero
if Pop. III binaries are always wide ($\gtrsim 100 R_\odot$), and if Pop. III
stars always enter into chemically homogeneous evolution. The presence of close
Pop. III binaries ($\sim 10 R_\odot$) are crucial for avoiding the worst
scenario.Other Link： http://arxiv.org/pdf/2008.01890v3
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Early Evolution of Disk, Outflow, and Magnetic Field of Young Stellar Objects: Impact of Dust Model
Tsukamoto, Y., Machida, M. N., Susa, H., Nomura, H., Inutsuka, S.
The Astrophysical Journal 2020.6
The formation and early evolution of low-mass young stellar objects (YSOs) are investigated using three-dimensional non-ideal magnetohydrodynamics simulations. We investigate the evolution of YSOs up to $\sim {10}^{4}\,\mathrm{yr}$ after protostar formation, at which protostellar mass reaches $\sim 0.1{M}_{\odot }$ . We particularly focus on the impact of the dust model on the evolution. We found that a circumstellar disk is formed in all simulations, regardless of the dust model. Disk size is approximately 10 au at the protostar formation epoch, and it increases to several tens of au at $\sim {10}^{4}\,\mathrm{yr}$ after protostar formation. The disk mass is comparable to the central protostellar mass, and gravitational instability develops. In simulations with small dust sizes, the warp of the pseudodisk develops $\sim {10}^{4}\,\mathrm{yr}$ after protostar formation. The warp strengthens magnetic braking in the disk and decreases disk size. Ion-neutral drift can occur in the infalling envelope when the typical dust size is $a\gtrsim 0.2\,\mu {\rm{m } }$ and the protostar (plus disk) mass is $M\gtrsim 0.1{M}_{\odot }$ . The outflow activity is anticorrelated to the dust size, and the strong outflow appears with small dust grains....
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Driving conditions of protostellar outflows in different star-forming environments Reviewed
Koki Higuchi, Masahiro N Machida, Hajime Susa
Monthly Notices of the Royal Astronomical Society 486 ( 3 ) 3741 - 3754 2019.7
Joint Work
Other Link： https://academic.oup.com/mnras/article-abstract/486/3/3741/5472907?redirectedFrom=fulltext
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Blocking metal accretion onto low-mass population III stars by stellar wind
Tanaka S.J, Chiaki G, Tominaga N, Susa H.
AIP Conference Proceedings 1947 2018.4
Joint Work
Low-mass population III (PopIII) stars could survive up until the present. They would be found in the halo of Our Galaxy, if they exist. Non-detection of low-mass PopIII stars in our Galaxy suggests that PopIII stars have a top-heavy IMF. On the other hand, some claims that the lack of such stars is as a result of metal enrichment of their surface by accretion of heavy elements from interstellar medium (ISM). We investigate effects of the stellar wind on the metal accretion onto low-mass PopIII stars because accretion of the local ISM onto the Sun is prevented by the solar wind even for neutrals. We find that low-mass PopIII stars traveling across the Galaxy forms the stellar magnetosphere in most of their life. Once the magnetosphere is formed, most of neutral interstellar particles are photoionized before reaching to the stellar surface and are blown away by the wind. This demonstrates that low-mass PopIII stars remain pristine and will be found as metal free stars and that further searches for them are valuable to constrain the IMF of PopIII stars....
DOI： 10.1063/1.5030805
Other Link： http://orcid.org/0000-0002-3380-5302
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Evolution of magnetic fields in collapsing star-forming clouds under different environments Reviewed
Koki Higuchi, Masahiro N Machida, Hajime Susa
Monthly Notices of the Royal Astronomical Society 475 ( 3 ) 3331 - 3347 2018.1
Joint Work
DOI： 10.1093/mnras/sty046
Other Link： https://academic.oup.com/mnras/article-abstract/475/3/3331/4795331?redirectedFrom=PDF
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Blocking Metal Accretion onto Population III Stars by Stellar Wind Reviewed
Tanaka, Shuta J.; Chiaki, Gen; Tominaga, Nozomu; Susa, Hajime
The Astrophysical Journal 844 ( 2 ) id.137 2017.8
Joint Work
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Formation environment of Pop II stars affected by the feedbacks from Pop III stars
Chiaki G, Susa H, Hirano S.
Memorie della Societa Astronomica Italiana - Journal of the Italian Astronomical Society 88 ( 4 ) 856 - 857 2017
Joint Work
Stars with metallicities abH Fe < -3 are called extremely metal-poor (EMP) stars, and considered to be formed in clouds enriched with metal from a single or several supernovae (SNe) of the first-generation (Pop III) stars. To confirm this, we numerically follow the enrichment process of minihalos (MHs) which have hosted Pop III stars. During their main-sequence (MS), the ionizing photons can not or partly break the gas around the Pop III stars because the halo binding energy is marginally larger than the radiation energy. After SN explosions, the gas continues to accrete along filaments of the large-scale structures, and the gas collapses again in the MHs within ̃ 10 Myr for low-mass MHs (3E 5 M☉) while ̃ 1 Myr for massive MHs (3E 6 M☉). The metallicity in the recollapsing regions is 10-4-10-2 Z☉ /SUB> and 10-6-10-5 Z<SUB>☉, respectively. This indicates that EMP stars are formed in the clouds enriched by a single SN in low-mass MHs....
Other Link： http://orcid.org/0000-0002-3380-5302
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Stellar wind prevents the ISM gas from accreting onto the Pop III stars
Susa H, Tanaka S, Chiaki G, Tominaga N.
Memorie della Societa Astronomica Italiana 88 ( 4 ) 862 - 863 2017
Joint Work
Hyper Metal Poor (HMP) stars in our Galactic halo are often interpreted as the population III (pop III) stars covered by a slight amount of accreted heavy elements from ISM. However, in the case of the Sun, the solar wind maintains the heliosphere which prevents the ISM gas from accreting. We assess the effects of the stellar wind from main sequence pop III stars on the metal accretion onto the surface of them. As a result, we find the heavy elements are ionized by the stellar radiation to be picked up by the stellar wind, thus they cannot touchdown the stellar surface. This result strongly suggests that the HMP stars were born from the gas with a slight amount of metals, which preserves the information of the progenitor stars....
Other Link： http://orcid.org/0000-0002-3380-5302
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Dissipation of Magnetic Fields in Star-forming Clouds with Different Metallicities Reviewed
Susa, Hajime; Doi, Kentaro; Omukai, Kazuyuki
The Astrophysical Journal 801 ( 1 ) id.13 2015.3
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On the IMF of first stars Reviewed
Susa, H.; Hasegawa, K.; Tominaga, N.
AIP Conference Proceedings 1594 ( 1 ) 105 - 108 2014.5
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Modeling Line-Driven Disk Wind for Broad Absorption Lines of Quasars Reviewed
Nomura, Mariko; Ohsuga, Ken; Wada, Keiichi; Susa, Hajime; Misawa, Toru
Publications of the Astronomical Society of Japan 65 ( 2 ) id.40 2014.4
Joint Work
DOI： 10.1093/pasj/65.2.40
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Generation of Magnetic Field on the Accretion Disk around a Proto-first-star Reviewed
Shiromoto, Yuki; Susa, Hajime; Hosokawa, Takashi
The Astrophysical Journal 782 ( 2 ) id.108 2014.2
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Fluid simulation of plume head-on collision dynamics during pulsed laser ablation Reviewed
Fujii, R.; Doi, K.; Yokoyama, Y.; Fukuoka, H.; Sugimura, A.; Tominaga, N.; Susa, H.; Umezu, I
Journal of Physics: Conference Series 454 ( 1 ) 9d.012009 2013.8
Joint Work
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Formation and radiative feedback of first objects and first galaxies Reviewed
Umemura, Masayuki; Susa, Hajime; Hasegawa, Kenji; Suwa, Tamon; Semelin, Benoit
Progress of Theoretical and Experimental Physics 2012 ( 1 ) id.01A306 2012.10
Joint Work
DOI： 10.1093/ptep/pts018
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Radiative feedback from proto-first-stars
Susa, Hajime
FIRST STARS IV - FROM HAYASHI TO THE FUTURE -. AIP Conference Proceedings 1480 67 - 70 2012.9
Single Work
DOI： 10.1063/1.4754330
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Dissipation of magnetic fields in low-metallicity clouds Show affiliations
Doi, Kentaro; Susa, Hajime; Omukai, Kazuyuki
FIRST STARS IV - FROM HAYASHI TO THE FUTURE -. AIP Conference Proceedings 1480 349 - 351 2012.9
Joint Work
DOI： 10.1063/1.4754383
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Effects of Photodissociation on the Fragmentation of Accretion Disk around Proto First Stars
Susa, H
ASP Conference Proceedings 458 81 2012.8
Single Work
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Effects of Photodissociation on the Fragmentation of Accretion Disk around Proto First Stars
Galactic Archaeology: Near-Field Cosmology and the Formation of the Milky Way 2012.8
Single Work
Very recently, the final stages of the first star formation have been investigated intensively. At such stages, an accretion disk is formed around the very first seed of a proto-star, and they seem to fragment inevitably. As a result, it might be possible for sub-solar mass first stars to form from such fragments. However, most of the previous numerical studies did not include the effects of photodissociating radiation from the first proto-stars, which could suppress the fragmentation of the disk. In this paper we include the transfer of Lyman-Werner photons to assess the effects of photodissociation on the fragmentation of the accretion disk. Consequently, we find that the dissociating radiation tends to suppress the mass accretion, but it cannot quench the fragmentation process....
Other Link： http://orcid.org/0000-0002-3380-5302
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Line-Driven Disk Wind Model for BALQSOs
AGN Winds in Charleston 2012.8
Single Work
We investigate the conditions for which the line-driven disk wind model reproduces broad absorption line (BAL) features. We research the structure of the disk wind using non-hydrodynamic calculations for a wide range of the parameters. We compare the resulting wind structure with the wind properties of X-ray observations of BAL quasars. We find that the probability that the BAL is observed is large for large black hole mass and large Eddington ratio. When the density of gas at the wind-base is the same as that of the broad emission-line region (BELR), the disk wind model can reproduce BAL features. Our results support the new hypothesis that the wind-base is the BELR....
Other Link： http://orcid.org/0000-0002-3380-5302
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SPH Radiative Hydrodynamics Methods Invited Reviewed
Hajime Susa
Proceedings of the International Astronomical Union 6 195 - 202 2011.4
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SPH radiative hydrodynamics methods
Susa H.
Proceedings of the International Astronomical Union 6 ( S270 ) 195 - 202 2011.4
Single Work
In this paper, we review the radiative hydrodynamics methods based upon Smoothed Particle Hydrodynamics(SPH). There are already various implementations so far, which can be categorized into three types: moment equation solvers, Monte Carlo methods, and ray-tracing schemes. These codes have been applied to various astrophysical problems including dynamics of dense proto-stellar cores, photoionization feedback of massive stars on molecular clouds, radiative feedback in the early universe, etc. Among these different methods, we focus on the ray-tracing schemes. We also describe one particular ray-tracing code RSPH in some details....
DOI： 10.1017/S1743921311000366
Other Link： http://orcid.org/0000-0002-3380-5302
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Generation of Seed Magnetic Field Around First Stars: Effects of Radiation Force Reviewed
Masashi Ando, Kentaro Doi, Hajime Susa
The Astrophysical Journal 716 ( 2 ) 1566 - 1572 2010.11
Joint Work
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Generation of Magnetic Fields by Radiation Pressure from the First Stars
Hajime Susa, Masashi Ando, Kentaro Doi
AIP Conference Proceedings 1294 291 - 292 2010.11
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Generation of magnetic fields by radiation pressure from the first stars
Susa H, Ando M, Doi K.
AIP Conference Proceedings 1294 291 - 292 2010.11
Joint Work
We investigate seed magnetic field generation by radiation pressure from the first stars. We include induction effects by electromagnetic fields, which were not taken into account previously. As a result, we find much smaller magnetic fields than predicted by the static H II regions assumed in our earlier models, even if momentum transfer due to photoionizations, which were excluded from our previous work, is present. The resultant magnetic flux density around the first star is as large as 10-20 G. These seed magnetic field will not affect subsequent star formation in the neighborhood of the first stars....
DOI： 10.1063/1.3518883
Other Link： http://orcid.org/0000-0002-3380-5302
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Generation of Seed Magnetic Field around First Stars
Masashi Ando, Kentaro Doi, Hajime Susa
AIP Conference Proceedings 1238 133 - 135 2010.6
Joint Work
DOI： 10.1063/1.3455916
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The Collapse of First Objects driven by Dark Matter Cusps
Masayuki Umemura, Suwa Tamon, Hajime Susa
AIP Conference Proceedings 1238 101 - 106 2010.6
Joint Work
DOI： 10.1063/1.3455909
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Radiative Regulation of Population III Star Formation
須佐元
Monthly Notices of the Royal Astronomical Society 2009.11
Single Work
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Feedback Effects of First Supernovae on the Neighboring Dark Matter Halos Reviewed
Masaru Sakuma, Hajime Susa
The Astrophysical Journal 698 155 2009.11
Joint Work
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Cosmological radiative transfer comparison project - II. The radiation-hydrodynamic tests Reviewed
Ilian Iliev et.al.
Monthly Notices of the Royal Astronomical Society 400 ( 3 ) 1283 - 1316 2009.11
Joint Work
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Formation Criteria and the Mass of Secondary Population III Stars Reviewed
Hajime Susa, Masayuki Umemura, Kenji Hasegawa
The Astrophysical Journal 702 ( 1 ) 480 - 488 2009.11
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Feedback effects of first supernovae on the neighboring dark matter halos
Sakuma M, Susa H.
Astrophysical Journal 698 ( 1 ) 155 - 162 2009.6
Joint Work
The first-generation stars in the Λ cold dark matter universe are considered to have formed in dark halos with total masses in the range ~105-107 M sun at z ~ 20-50. These stars expected to be very massive and in some cases they end their lives as the first supernovae (SNe). We explore the problem of whether star formation in low-mass dark halos (<=107 M sun) was triggered or suppressed by the SN feedback from neighboring star-forming halos. We take into consideration mainly two effects by the SN shock: one is the evacuation of gas components from the halos and the other is the promotion of H2 formation because of the enhanced ionization degree by shock heating. Combining above effects, we find that the star formation activities in the neighboring dark matter halos (M <= 107 M sun) are basically suppressed in case they are located close to the SN center, because of the gas evacuation effect. The critical distance within which the gas is blown away falls within the range ~0.3-1.5 kpc depending on the SN energy and the halo mass. In addition, we find there is very little window in the parameter space where star formation activities in dark halos are induced or promoted by neighboring SN....
DOI： 10.1088/0004-637X/698/1/155
Other Link： http://orcid.org/0000-0002-3380-5302
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Radiative regulation of population III star formation
Hasegawa K, Umemura M, Susa H.
Monthly Notices of the Royal Astronomical Society 395 ( 3 ) 1280 - 1286 2009.5
Joint Work
We explore the impact of ultraviolet (UV) radiation from massive Population III (Pop III) stars of 25, 40, 80 and 120Msolar on the subsequent Pop III star formation. In this paper, particular attention is paid to the dependence of radiative feedback on the mass of source Pop III star. UV radiation from the source star can work to impede the secondary star formation through the photoheating and photodissociation processes. Recently, Susa and Umemura have shown that the ionizing radiation alleviates the negative effect by H2-dissociating radiation from 120Msolar Pop III star, since an H2 shell formed ahead of an ionizing front can effectively shield H2-dissociating radiation. On the other hand, it is expected that the negative feedback by H2-dissociating radiation can be predominant if a source star is less massive, since a ratio of the H2-dissociating photon number to the ionizing photon number becomes higher. In order to investigate the radiative feedback effects from such less massive stars, we perform three-dimensional radiation hydrodynamic simulations, incorporating the radiative transfer effect of ionizing and H2-dissociating radiation. As a result, we find that if a source star is less massive than ~25Msolar, the ionizing radiation cannot suppress the negative feedback of H2-dissociating radiation. Therefore, the fate of the neighbouring clouds around such less massive stars is determined solely by the flux of H2-dissociating radiation from source stars. By making analytic estimates of H2 shell formation and its shielding effect, we derive the criteria for radiation hydrodynamic feedback depending on the source star mass....
DOI： 10.1111/j.1365-2966.2009.14639.x
Other Link： http://orcid.org/0000-0002-3380-5302
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FIRST Project: Formation and Feedback of the First Stars
Masayuki Umemura, Hajime Susa, Suwa Tamon, Daisuke Sato
AIP Conference Proceedings 990 386 - 389 2009.3
Joint Work
DOI： 10.1063/1.2905587
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Swinyard B, Nakagawa T, Merken P, Royer P, Souverijns T, Vandenbussche B, Waelkens C, Davis P, Di Francesco J, Halpern M, Houde M, Johnstone D, Joncas G, Naylor D, Plume R, Scott D, Abergel A, Bensammar S, Braine J, Buat V, Burgarella D, Cais P, Dole H, Duband L, Elbaz D, Gerin M, Giard M, Goicoechea J, Joblin C, Jones A, Kneib J.P, Lagache G, Madden S, Pons R, Pajot F, Rambaud D, Ravera L, Ristorcelli I, Rodriguez L, Vives S, Zavagno A, Geis N, Krause O, Lutz D, Poglitsch A, Raab W, Stegmaier J, Sturm E, Tuffs R, Lee H.M, Koo B.-C, Im M, Pak S, Han W, Park J.-H, Nam U.-W, Jin H, Lee D.-H, Yuk I.-S, Lee S, Aikawa Y, Arimoto N, Doi Y, Enya K, Fukagawa M, Furusho R, Hasegawa S, Hayashi M, Honda M, Ida S, Imanishi M, Inutsuka S.-I, Izumiura H, Kamaya H, Kaneda H, Kasuga T, Kataza H, Kawabata K, Kawada M, Kawakita H, Kii T, Koda J, Kodama T, Kokubo E, Komatsu K, Matsuhara H, Matsumoto T, Matsuura S, Miyata T, Murakam H, Nagata H, Nagata T, Nakajima T, Naoto K, Nishi R, Noda A, Okamoto A, Okamoto Y.K, Omukai K, Onaka T, Ootsubo T, Ouchi M, Saito H, Sato Y, Sako S, Sekiguchi T, Shibai H, Sugita H, Sugitani K, Susa H, Tae-soo P, Tamura M, Ueda Y, Ueno M, Wada T, Watanabe J, Yamada T, Yamamura I, Yoshida N, Yoshimi K, Yui Y, Benedettini M, Cerulli R, Di Giorgio A, Molinari S, Orfei R, Pezzuto S, Piazzo L, Saraceno P, Spinoglio L, de Graauw T, de Korte P, Helmich F, Hoevers H, Huisman R, Shipman R, van der Tak F, van der Werf P, Wild W, Acosta-Pulido J, Cernicharo J, Herreros J, Martin-Pintado J, Najarro F, Perez-Fourmon I, Pardo J.R, Gomez F, Rodriguez N.C, Ade P, Barlow M, Clements D, Ferlet M, Fraser H, Griffin D, Griffin M, Hargrave P, Isaak K, Ivison R, Mansour M, Laniesse J, Mauskopf P, Morozov D, Oliver S, Orlando A, Page M, Popescu C, Serjeant S, Sudiwala R, Rigopoulou D, Walker I, White G, Viti S, Winter B, Bock J, Bradford M, Harwit M, Holmes W.
Experimental Astronomy 23 ( 1 ) 193 - 219 2009.3
Joint Work
The Space Infrared telescope for Cosmology and Astrophysics (SPICA) is planned to be the next space astronomy mission observing in the infrared. The mission is planned to be launched in 2017 and will feature a 3.5 m telescope cooled to <5 K through the use of mechanical coolers. These coolers will also cool the focal plane instruments thus avoiding the use of consumables and giving the mission a long lifetime. SPICA's large, cold aperture will provide a two order of magnitude sensitivity advantage over current far infrared facilities (>30 microns wavelength). We describe the scientific advances that will be made possible by this large increase in sensitivity and give details of the mission, spacecraft and focal plane conceptual design....
DOI： 10.1007/s10686-008-9090-0
Other Link： http://orcid.org/0000-0002-3380-5302
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Regulated star formation in forming disk galaxies exposed to the
須佐元
Astrophysical Journal Volume 684 ( 226 ) 2008.11
Single Work
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Regulated star formation in forming disk galaxies exposed to the ultraviolet radiation background
Susa H.
Astrophysical Journal 684 ( 1 ) 226 - 235 2008.9
Single Work
We perform radiation hydrodynamics simulations on the evolution of galactic gas disks irradiated by the ultraviolet radiation background. We find that gas disks with NHgtrsim 1021 cm-2 that are exposed to ultraviolet radiation at a level of I21 = 1 can be self-shielded from photoheating, whereas disks with NHlesssim 1021 cm-2 cannot. We also find that the unshielded disks keep a smooth density distribution without any sign of fragmentation, while the self-shielded disks easily fragment into small pieces through self-gravity, possibly followed by star formation. The suppression of star formation in unshielded disks is different from the photoevaporation effect, since the assumed dark halo potential is deep enough to retain the photoheated gas. The presence of such a critical threshold column density would be one of the reasons for the so-called downsizing feature of present-day galaxies....
DOI： 10.1086/589964
Other Link： http://orcid.org/0000-0002-3380-5302
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Radiative Transfer SPH Simulations of UV Feedback on Pop III Star Formation
Daisike Sato, Masayuki Umemura, Hajime Susa, Suwa Tamon
AIP Conference Proceedings 990 393 - 394 2008.3
Joint Work
DOI： 10.1063/1.2905589
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High Resolution P3M-GRAPE-SPH Simulations of PopIII Star Formation
Suwa Tamon, Masayuki Umemura, Daisike Sato, Hajime Susa,
AIP Conference Proceedings 990 390 - 392 2008.3
Joint Work
DOI： 10.1063/1.2905588
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Radiative transfer SPH Simulations of UV feedback on pop III star formation
Sato D, Umemura M, Susa H, Suwa T.
AIP Conference Proceedings 990 393 - 394 2008.3
Joint Work
To explore the radiative feedback from first stars on the subsequent Pop III star formation in a Lambda-CDM universe, we perform high-resolution cosmological SPH simulations coupled with radiative transfer of UV radiation, using FIRST simulator. In the simulations, the baryon mass resolution is 0.3 Msolar and the number of particles is several 105 respectively for baryon and dark matter component. We assume that several collapsing cores form in a Pop III object and one of them evolves into a first star, and then investigate the evolution of other collapsing cores with paying special attention to the radiation hydrodynamic feedback. As a result, it is shown that other collapsing cores are impervious to the negative radiative feedbacks of H2 photodissociation and photoevapolation, but can continue to collapse due to the shielding of UV radiation, consistent with the results of Susa & Umemura (2006). This result implies that all collapsing cores in a Pop III object can evolve into stars....
DOI： 10.1063/1.2905589
Other Link： http://orcid.org/0000-0002-3380-5302
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Instability of Population III Black Hole Accretion Disks Reviewed
Ken Ohsuga, Hajime Susa, Yosuke Uchiyama
Publications of the Astronomical Society of Japan 59 ( 6 ) 1235 - 1241 2007.12
Joint Work
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Instability of population III black hole accretion disks
Ohsuga K, Susa H, Uchiyama Y.
Publications of the Astronomical Society of Japan 59 ( 6 ) 1235 - 1241 2007.12
Joint Work
We investigated the stability of black hole accretion disks in a primordial environment (POP III disks for short), by solving the vertical structure of optically thick disks, including convective energy transport, and by employing a one-zone model for optically thin isothermal disks. Because of the absence of metals in POP III disks, we find significant differences in stability associated with ionization between POP III disks and the disks of solar metallicity. An unstable branch in S-shaped equilibrium curves on the dot{M} - Σ (mass accretion rate-surface density) plane extends to a larger surface density compared with the case of disks of solar metallicity. The resulting equilibrium loci indicate that quasi-periodic oscillations in luminosity can also be driven in POP III disks, and their maximal luminosity is typically by an order of magnitude larger than that of the disks of solar metallicity. Such a strong outburst of POP III disks can be observed by future huge telescopes, in the case that the mass is supplied onto the disks at the Bondi accretion rates in typical virialized small dark halos....
Other Link： http://orcid.org/0000-0002-3380-5302
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Photodissociation feedback of Population III stars on their neighbor prestellar cores
須佐元
Astrophysical Journal Volume 659 ( 908 ) 2007.11
Single Work
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Dissipation of Magnetic Flux in Primordial Star Formation: From Run-away phase to Mass Accretion Phase
須佐元
Publication of Astronomical Society of Japan Volume 59 ( 787 ) 2007.11
Single Work
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Dissipation of Magnetic Flux in Primordial Star Formation: From Run-away Phase to Mass-Accretion Phase
Publications of the Astronomical Society of Japan 2007.8
Single Work
We investigate the dissipation of magnetic flux in primordial star-forming clouds throughout their collapse including the run-away collapse phase as well as the accretion phase. We solve the energy equation and the non-equilibrium chemical reactions in the collapsing gas, in order to obtain the radial distribution of the ionized fraction during the collapse. As a result, we find the ionized fraction is high enough for the magnetic field to couple with the gas throughout the evolution of the cloud. This result suggests that the jet formation from protostars as well as the activation of magneto-rotational instability in the accretion disk are enabled in the presence of the cosmological seed magnetic flux proposed by Langer et al.(2003)....
Other Link： http://orcid.org/0000-0002-3380-5302
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Radiative feedback of POPIII stars Invited
Hajime Susa
Published online at http://www.arcetri.astro.it/hisur.sarticle #6 2007.6
Single Work
Authorship：Lead author
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Radiative feedback of POPIII stars
HI Survival Through Cosmic Times 2007.6
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Photodissociation feedback of population III stars onto neighboring prestellar cores
Susa H.
Astrophysical Journal 659 ( 2 I ) 908 - 917 2007.4
Single Work
We investigate the star formation process in the primordial environment in the presence of radiative feedback from other Population III stars that formed earlier. In this paper, we focus our attention on the effects of photodissociative radiation, leading toward a full understanding of the radiative feedback effects. We perform three-dimensional radiation hydrodynamics simulations on this issue, as well as analytic estimates, paying special attention to the self-shielding effect and the dynamics of the star-forming cloud. As a result, we find that the ignition timing of the source star is crucial. If the ignition is later than the epoch when the central density of the collapsing cloud exceeds ~103-104 cm-3, the collapse cannot be reversed, even if the source star is located at <~100 pc. The uncertainty of the critical density comes from the variety of initial conditions of the collapsing cloud. We also find the analytic criterion for a cloud to collapse with a given central density, temperature, and Lyman-Werner-band flux that irradiates the cloud. Although we focus on the radiation from neighboring stars, this result can also be applied to the effects of the diffuse Lyman-Werner (LW) radiation field that is expected to be built up prior to the reionization of the universe. We find that self-gravitating clouds can easily self-shield from diffuse LW radiation and continue their collapse for densities larger than ~103 cm-3....
DOI： 10.1086/512670
Other Link： http://orcid.org/0000-0002-3380-5302
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Maki H, Susa H.
Publications of the Astronomical Society of Japan 59 ( 4 ) 786 - 797 2007
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Cosmological Radiative Transfer Codes Comparison Project I: The Static Density Field Tests
須佐元
Monthly Notice of the Royal Astronimical Society 371 ( 1057 ) 2006.11
Single Work
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Secondary Star Formation in a Population III Object
須佐元
Astrophysical Journal Volume 645 ( L93 ) 2006.11
Single Work
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Cosmological radiative transfer codes comparison project - I. The static density field tests
Iliev I.T, Ciardi B, Alvarez M.A, Maselli A, Ferrara A, Gnedin N.Y, Mellema G, Nakamoto T, Norman M.L, Razoumov A.O, Rijkhorst E.-J, Ritzerveld J, Shapiro P.R, Susa H, Umemura M, Whalen D.J.
Monthly Notices of the Royal Astronomical Society 371 ( 3 ) 1057 - 1086 2006.9
Joint Work
Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal HII region expansion; (2) HII region expansion with evolving temperature; (3) I-front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form....
DOI： 10.1111/j.1365-2966.2006.10775.x
Other Link： http://orcid.org/0000-0002-3380-5302
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Environment Surrounding the First Stars
Hajime Susa
Online at http://www.int.washington.edu/talks/WorkShops/int_06_2a 2006.7
Single Work
Authorship：Lead author
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Environment Surrounding the First Stars
The First Stars and Evolution of the Early Universe 2006.7
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Secondary star formation in a population III object
Susa H, Umemura M.
Astrophysical Journal 645 ( 2 II ) 2006.7
Joint Work
We explore the possibility of subsequent star formation after a first star forms in a Population III object, by focusing on the radiation-hydrodynamic (RHD) feedback caused by ionizing photons, as well as H2-dissociating photons. For this purpose, we perform three-dimensional RHD simulations in which the radiative transfer of ionizing photons and H2-dissociating photons from a first star is self-consistently coupled with hydrodynamics based on a smoothed particle hydrodynamics method. It is shown that density peaks above a threshold density can keep collapsing, owing to the shielding of H2-dissociating radiation by an H2 shell formed ahead of a D-type ionization front. But, below the threshold density an M-type ionization front with a shock propagates, and density peaks are radiation-hydrodynamically evaporated by the shock. The threshold density depends on the distance from the source star and is ~102 cm-3 for a source distance of 30 pc. Taking into consideration that the extent of a Population III object is ~100 pc and the density peaks within it have densities of 102-104 cm-3, it is concluded that secondary star formation is possible in the broad regions of a Population III object....
DOI： 10.1086/506275
Other Link： http://orcid.org/0000-0002-3380-5302
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Smoothed particle hydrodynamics coupled with radiation transfer
Susa H.
Publications of the Astronomical Society of Japan 58 ( 2 ) 445 - 460 2006.4
Single Work
We have constructed a brand-new radiation hydrodynamics solver based upon Smoothed Particle Hydrodynamics, which works on a parallel computer system. The code is designed to investigate the formation and evolution of first-generation objects at z ≳ 10, where the radiative feedback from various sources plays important roles. The code can compute the fraction of chemical species e, H+, H, H-, H2, and H+2 by by fully implicit time integration. It also can deal with multiple sources of ionizing radiation, as well as radiation at Lyman-Werner band. We compare the results for a few test calculations with the results of one-dimensional simulations, in which we find good agreements with each other. We also evaluate the speedup by parallelization, which is found to be almost ideal, as long as the number of sources is comparable to the number of processors....
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation of dwarf galaxies in reionized universe with heterogeneous multicomputer system
Boku T, Susa H, Onuma K, Umemura M, Sato M, Takahashi D.
International Journal for Multiscale Computational Engineering 4 ( 2 ) 281 - 289 2006
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The effects of early reionization on the formation of dwarf galaxies
Hajime Susa, Masayuki Umemura
IAU Colloquium Proceedings of the international Astronomical Union 198 147 - 150 2005.11
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Dissipation of Magnetic Fields in Primordial Gas Clouds
Hideki Maki, Hajime Susa
AIP Conference Proceedings 784 808 - 818 2005.9
Joint Work
DOI： 10.1063/1.2077254
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Formation of Dwarf Galaxies during the Cosmic Reionization
須佐元
Astrophysical Journal Volume 600 ( 1 ) 2004.11
Single Work
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Dissipation of Magnetic Flux in Primordial Clouds
須佐元
Astrophysical Journal Volume 609 ( 467 ) 2004.11
Single Work
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The Effects of Early Cosmic Reionization on the Substructure Problem in Galactic Halos
須佐元
Astrophysical Journal Volume 610 ( L5 ) 2004.11
Single Work
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The structure and evlution of early cosmological HII regions
須佐元
Astrophysical Journal Volume 613 ( 631 ) 2004.11
Single Work
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The structure and evolution of early cosmological H II regions
Kitayama T, Yoshida N, Susa H, Umemura M.
Astrophysical Journal 613 ( 2 I ) 631 - 645 2004.10
Joint Work
We study the formation and evolution of H II regions around the first stars formed at redshifts z=10-30. We use a one-dimensional Lagrangian hydrodynamics code that self-consistently incorporates radiative transfer and nonequilibrium primordial gas chemistry. The star-forming region is defined as a spherical dense molecular gas cloud with a Population III star embedded at the center. We explore a large parameter space by considering, as plausible early star-forming sites, dark matter halos of mass Mhalo=105-108 Msolar, gas density profiles with a power-law index w=1.5-2.25, and metal-free stars of mass Mstar=25-500 Msolar. The formation of the H II region is characterized by initial slow expansion of a weak D-type ionization front near the center, followed by rapid propagation of an R-type front throughout the outer gas envelope. We find that the transition between the two front types is indeed a critical condition for the complete ionization of halos of cosmological interest. In small-mass (<~106 Msolar) halos, the transition takes place within a few 105 yr, yielding high escape fractions (>80%) of both ionizing and photodissociating photons. The gas is effectively evacuated by a supersonic shock, with the mean density within the halo decreasing to <~1 cm-3 in a few million years. In larger mass (>~107 Msolar) halos, the ionization front remains to be of D-type over the lifetime of the massive star, the H II region is confined well inside the virial radius, and the escape fractions are essentially zero. We derive an analytic formula that reproduces well the results of our simulations for the critical halo mass below which the gas is completely ionized. We discuss immediate implications of the present results for the star formation history and early reionization of the universe....
DOI： 10.1086/423313
Other Link： http://orcid.org/0000-0002-3380-5302
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Dissipation of magnetic flux in primordial gas clouds
Maki H, Susa H.
Astrophysical Journal 609 ( 2 I ) 467 - 473 2004.7
Joint Work
We report on the strength of the seed magnetic flux of the accretion disk surrounding Population III stars. Themagnetic field in accretion disks might play an important role in the transport of angular momentum because of the turbulence induced by magnetorotational instability (MRI). On the other hand, since the primordial star-forming clouds contain no heavy elements or grains, they experience a very different thermal history and magnetic-field-dissipation history in the course of their gravitational contraction from those in the present-day star-forming molecular clouds. In order to assess the magnetic field strength in the accretion disk of Population III stars, we calculate the thermal history of the primordial collapsing clouds and investigate the coupling of the magnetic field with the primordial gas. As a result, we find that the magnetic field strongly couples with the primordial gas cloud throughout the collapse, i.e., the magnetic field is frozen to the gas, as long as the initial field strength satisfies B<~10-5(NH/103cm-3)0.55G....
DOI： 10.1086/421103
Other Link： http://orcid.org/0000-0002-3380-5302
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Quasar Mesolensing as a Probe of CDM Substructures
Dark Matter in Galaxies 2004.7
Single Work
We investigate methods to detect numerous CDM substructures around galaxies predicted by cosmological N-body simulations via medium-scale gravitational-lens effect. Magnification anomaly due to such substructures have already been noted by some researchers and applied to gravitationally lensed quasars. In this presentation we focus on other phenomena produced by gravitational lens effects multiple images and the time delays between images at the same situation i.e. the lens is a galaxy and the source is a distant quasar. Here we have included the effect of the galaxy itself so-called ""Chang-Refsdal lens"" situation and estimated image separations and the time delays. The expected values are from 1 to 100 mas for the image separation and from 10 to 1000 sec for the time delay with slight lens model dependence. Event rate for such phenomena is the order of percent but such signals in one image of a gravitationally lensed quasar must be a direct evidence for the numerous CDM substructures. Image separations and time delays are reflect mass of the lens and our methods will provide better constraints on the substructres compared with magnification anomaly....
Other Link： http://orcid.org/0000-0002-3380-5302
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The effects of early cosmic reionization on the substructure problem in galactic halos
Susa H, Umemura M.
Astrophysical Journal 610 ( 1 II ) 2004.7
Joint Work
Recent observations of the cosmic microwave background by the Wilkinson Microwave Anisotropy Probe strongly suggest that the reionization of the universe took place quite early (z~17). On the other hand, it has been pointed out that the cold dark matter cosmology suffers from a substructure problem in which more subgalactic halos are produced in the Local Group than dwarf galaxies. In this Letter, as a potential mechanism for solving this problem, we consider the feedback effects of early reionization on the formation of small-scale structures. For this purpose, we perform three-dimensional radiation hydrodynamic simulations, incorporating the radiative transfer for ionizing photons. As a result, it is found that the early reionization is devastating for low-mass systems with Mvir<~108 Msolar or vcirc<~20 km s-1, and almost all gas is photoevaporated in more than 95% of low-mass systems. Such a strong negative feedback on the formation of low-mass galaxies may solve the substructure problem and support the picture that Local Group dwarf galaxies are descendants of the more massive halos that experienced and survived tidal stripping....
DOI： 10.1086/423199
Other Link： http://orcid.org/0000-0002-3380-5302
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Galaxy Formation in the Presence of Ultraviolet Background Radiation Field --- A Simulation by Radiation-SPH ---
Hajime Susa, Masayuki Umemura
Proceedings of Japan-German Seminar, held in Sendai, Japan, July 24-28, 2001, Eds.: N. Arimoto and W. Duschl 83 - 86 2004.2
Joint Work
Authorship：Lead author
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Galaxy Formation in the Presence of Ultraviolet Background Radiation Field --- A Simulation by Radiation-SPH ---
Studies of Galaxies in the Young Universe with New Generation Telescope 2004.2
Single Work
We studied galaxy formation with Radiation-SPH simulations in order to find the effects of UVB field on the star formation in the forming galaxies. We find the UVB radiation field can delay the star formation in protogalactic clouds, especially at low redshift (z < 2). This result suggests that galaxies formed at lower redshift tend to be late type galaxies and early type galaxies tend to be formed at higher redshift....
Other Link： http://orcid.org/0000-0002-3380-5302
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The Epoch of Cosmic Reionization
Studies of Galaxies in the Young Universe with New Generation Telescope 2004.2
Single Work
Full radiative transfer for ionizing photons in 3D space has been solved to pursue the reionization history in an inhomogeneous universe. It has been shown that the inhomogeniety of the universe delays the reionization considerably. Also, QSO absorption line systems are simulated with using the results of radiative transfer calculations. Lyα continuum depression implies that the metagalactic UV intensity decreases rapidly with z at z > 5, and the reionization must have taken place between z = 6 and 10. Finally, it is stressed that Hα forest is a more powerful tool to probe the reionization history and the density fluctuations in the universe at z > 5....
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation of dwarf galaxies in reionized universe with heterogeneous multi-computer system
Boku T, Susa H, Onuma K, Umemura M, Sato M, Takahashi D.
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) 3039 629 - 636 2004
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Heterogeneous remote computing system for computational astrophysics with OmniRPC
Onuma K, Boku T, Sato M, Takahashi D, Susa H, Umemura M.
Proceedings - International Symposium on Applications and the Internet Workshops 623 - 629 2004
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Quasar Mesolensing - Direct Probe to Substructures around Galaxies
Yonehara A, Umemura M, Susa H.
Publications of the Astronomical Society of Japan 55 ( 6 ) 1059 - 1078 2003.12
Joint Work
Recently, a ``CDM crisis'' is being discussed. The main point of this crisis is that the number of substructures presented by cosmological N-body simulations based on the CDM scenario for structure formation is much larger than the observed substructures. Therefore, it is crucial for this crisis to discriminate whether the expected number of CDM substructures really exist without detectable radiation, or do not exist. In this paper, we present a new idea to detect such invisible substructures by utilizing gravitational lensing. Here, we consider quasars that are gravitationally lensed by a foreground galaxy. A substructure around a lensing galaxy may superposed on one of the lensed images of such quasars. In this situation, additional image splitting should occur in the image behind the substructure, and additional multiple images are created. From our estimations, the separation and time delay between the additional multiple images are expected to be 1-30 milli-arcsecond and 1-103 second, respectively. Furthermore, we evaluate that the optical depth for such events is sim 0.1. Consequently, future fine-resolution imaging and/or high-speed monitoring will unable us to find invisible substructures in one of a few multiple quasars....
Other Link： http://orcid.org/0000-0002-3380-5302
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Radiative transfer in astrophysics
Astronomical Herald 2003.12
Single Work
Lots of astrophysical issues are closely related to the problem of radiation transfer. In this paper, I try to outline the recent results on the cosmic reionization, and radiative feedback on the formation of galaxies, in which the multi-dimensional radiative tranfer problem is quite important. I also mention the future prospects on these issues....
Other Link： http://orcid.org/0000-0002-3380-5302
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Quasar Mesolensing - Direct Probe to Substructures around Galaxies
須佐元
Publications of the Astronomical Sciety of Japan 55 ( 1059 ) 2003.11
Single Work
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Radiation-Hydrodynamical Simulations of Primordial Galaxy Formation in the UV Background
Tetsu Kitayama, Hajime Susa, Masayuki Umemura, Satoru Ikeuchi
ASP Conference Proceedings 208 417 2003.11
Joint Work
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A Simulation of Galaxy Formation by Radiation-SPH
Hajime Susa, Masayuki Umemura
ASP Conference Proceedings 208 323 2003.11
Joint Work
Authorship：Lead author
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HMCS-G : Grid-enabled Hybrid Computing System for Computational Astrophysics (Grid Applications)
BOKU,TAISUKE, SATO,MITSUHISA, ONUMA,KENJI, MAKINO,JUNICHIRO, SUSA,HAJIME, TAKAHASHI,DAISUKE, UMEMURA,MASAYUKI
IPSJ Transactions on Computing Systems 44 ( 11 ) 1 - 13 2003.8
Joint Work
Publisher：Information Processing Society of Japan (IPSJ)
We have developed a hybrid computing system named HMCS-G which combines general purpose parallel systems and a special purpose machine for gravity calculation on computational grid environment. The prototype is implemented with GRAPE-6 gravity engine and OmniRPC as Grid-RPC with high portability and throughput. Through the preliminary performance evaluation on several network conditions, it is confirmed that the function and capability of HMCS-G can support actual application for multiphysics simulation. With HMCS-G, the utilization ratio of special purpose machines such as GRAPE-6 is also greatly enhanced through access from all over the world.
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Formation of Dwarf Galaxies in Reionized Universe
Hajime Susa, Masayuki Umemura
The Local Group as an Astrophysical Laboratory; poster papers from the Space Telescope Science Institute Symposium held in May, 2003. Edited by Mario Livio and Thomas M. Brown. Published by the Space Telescope Science Institute 94 2003.5
Joint Work
Authorship：Lead author
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Towards Direct Detection of Substructure around Galaxies -- Quasar Mesolensing
Atsunori Yonehara, Masayuki Umemura, Hajime Susa
ASP Conference Proceedings 289 496 - 472 2003.5
Joint Work
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Formation of Dwarf Galaxies in Reionized Universe
The Local Group as an Astrophysical Laboratory 2003.5
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Towards Direct Detection of Substructure around Galaxies -- Quasar Mesolensing
The Proceedings of the IAU 8th Asian-Pacific Regional Meeting, Volume 1 2003.5
Single Work
We discuss about detectability of substructure around galaxies by utilizing a quasar which is gravitationally lensed by an intervening galaxy. In this situation, background quasars have already shown multiple images. If a substructure in the lens galaxy is superposed on one of the images, additional gravitational lensing by the substructure will occur. The typical image separation due to the substructures with mass 10^8 M_o is on the order of micro-arcseconds. Even if we cannot resolve these images, the total flux variations of these images are expected to be observed, both as its own flux variation plus the ``echo"-like variation due to time delay between the images. The time delay of this ``echo"-like variation is estimated to be several tens of minutes....
Other Link： http://orcid.org/0000-0002-3380-5302
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Role of ultraviolet background radiation field on the formation of galaxies
Astronomical Herald 2003.2
Single Work
There are various types of galaxies in our universe. They are distributed over six orders of magnitude in mass, and have wide varieties of morphologies and colors. It is one of the main and far-reaching research themes in the present cosmology how these galaxies are formed and they come to have such diversity. These varieties might be caused by ultraviolet background radiation field (UVB) filled the universe in the era of galaxy formation, because UVB introduces a few physical scales to the formation processes of galaxies. In this paper, I will try to outline the effects of ultraviolet background radiation field on the formation of galaxies....
Other Link： http://orcid.org/0000-0002-3380-5302
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A Simulation of Galaxy Formation by Radiation-SPH
Astrophysical Supercomputing using Particle Simulations 2003
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HMCS-G: Grid-enabled hybrid computing system for computational astrophysics
Boku T, Sato M, Onuma K, Makino J, Susa H, Takahashi D, Umemura M, Ukawa A.
Proceedings - CCGrid 2003: 3rd IEEE/ACM International Symposium on Cluster Computing and the Grid 558 - 567 2003
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Radiation-Hydrodynamical Simulations of Primordial Galaxy Formation in the UV Background
Astrophysical Supercomputing using Particle Simulations 2003
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Miura H, Nakamoto T, Susa H.
Icarus 160 ( 2 ) 258 - 270 2002.12
Joint Work
A shock-wave heating model is one of the possible models for chondrule formation. We examine, within the framework of a shock-wave heating model, the effects of evaporation on the heating of chondrule precursor particles and the stability of their molten state in the postshock flow. We numerically simulate the heating process in the flow taking into account evaporation. We find that the melting criterion and the minimum radius criterion do not change significantly. However, if the latent heat cooling due to the evaporation dominates the radiative cooling from the precursor particle, the peak temperature of the precursor particle is suppressed by a few hundred Kelvins. We also find that the total gas pressure (ram plus static) acting on the precursor particle exceeds the vapor pressure of the molten precursor particle. Therefore, it is possible to form chondrules in the shock-wave heating model if the precursor temperature increases up to the melting point....
Other Link： http://orcid.org/0000-0002-3380-5302
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On the maximal size of chondrules in shock wave heating model
Susa H, Nakamoto T.
Astrophysical Journal 564 ( 1 II ) 2002.1
Joint Work
The shock wave heating model is one of the possible mechanisms that could form chondrules. Recently, it is suggested that the shock wave heating model can explain some chondrule features including the maximum temperature and cooling rates. Here we show that another two chondrule features, characteristic size and rotation rates, can be explained naturally, in the framework of the shock wave heating model. The sizes of chondrules are bounded by the balance between the surface tension and the ram pressure on a molten droplet. The acceptable upper bound is acrit~=1 mm. Moreover, when a chondrule forms through the disruption of a larger droplet, it acquires a spin of the order of ω~=1×103 rad s-1. Both features are consistent with observations....
DOI： 10.1086/338789
Other Link： http://orcid.org/0000-0002-3380-5302
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Hydrodynamics in astrophysics with gravity on Heterogeneous Multi-Computer System
T., Boku, J., Makino, H., Susa, M., Umemura, T., Fukushige, and, A. Ukawa
IPSJ Transactions on High Performance Computing Systems 41 5 2002.1
Joint Work
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A Shock-Wave Heating Model for Chondrule Formation: Peak Temperature of Precursor Particles and High Ambient Gas Pressure
8th Asian-Pacific Regional Meeting, Volume II 2002
Single Work
We examine, in a framework of the shock-wave heating model, the effects of evaporation on the heating of chondrule precursor particles and the stability of their molten state in the post-shock flow. What we have found are as follows; (1) latent heat cooling dominates radiative cooling when the temperature of the precursor particles is larger than Tc (where 1900 K <≈ Tc <≈ 2400 K), and decreases the peak temperature of the precursor particles by more than a few 100 K, and (2) the total gas pressure (ram plus static) acting on the precursor particles in the post-shock region is large enough to keep the molten precursor particles stable. Therefore, we can see that the shock-wave heating model satisfies some observational constraints for chondrule formation models (Jones et al. 2000), i.e., the peak temperature of the precursor particles and elevated ambient gas pressure....
Other Link： http://orcid.org/0000-0002-3380-5302
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Heterogeneous multi-computer system: A new platform for multi-paradigm scientific simulation
Boku T, Makino J, Susa H, Umemura M, Fukushige T, Ukawa A.
Proceedings of the International Conference on Supercomputing 26 - 34 2002
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Heterogeneous multi-computer system: A new paradigm of parallel processing
Boku T, Makino J, Susa H, Umemura M, Fukushige T, Ukawa A.
Proceedings - International Conference on Parallel Computing in Electrical Engineering, PARELEC 2002 3 - 8 2002
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Formation of Host Clouds of First Stars in the Early Universe
Susa H.
Progress of Theoretical Physics Supplement ( 147 SUPPL. ) 11 - 42 2002
Single Work
It is widely accepted that various structures of the universe are formed through the gravitational instability of the density perturbations imprinted by the quantum fluctuations. First stars are also formed as a consequence of the evolved density perturbations. However, they are not formed from the stellar size perturbations such as M ̃ 1-102 M☉. Recent theoretical investigations suggest the first stars are formed in more massive ``parent'' clouds, with M ≳ 106 M☉. The key physics of the formation of such small hosts is the formation of hydrogen molecules in them. Since the virial temperature of such clouds are less than 104 K, they are not cooled by atomic cooling processes. Therefore, hydrogen molecules are required as the main coolant of primordial gas at 104 K, because their rotation/vibration levels are excited even at such low temperature. In this chapter, gravitational collapse of density perturbations in Cold Dark Matter (CDM) universe is reviewed. Second, the formation processes of hydrogen molecules and the cooling mechanisms in primordial gas are summarized. Finally, the condition of the formation of cold dense gas clouds that host the first stars is shown....
DOI： 10.1143/PTPS.147.11
Other Link： http://orcid.org/0000-0002-3380-5302
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A shock-wave heating model for chondrule formation: evaporation effects on molten particles
AAS/Division for Planetary Sciences Meeting Abstracts #33 2001.11
Single Work
Chondrules are mm-sized, once-molten, spherical-shaped grains composed of silicate material in chondritic meteorites. It is considered that dust particles, which are chondrule precursors, were heated and melted, and cooled again to solidify in a short period of time, and formed chondrules, although the heating mechanism responsible for the chondrule formation has not been clearly understood yet. Shock-wave heating model is one of the popular models for the heating process. In the model, dust grains are heated due to the drag heating in the post shock flow. Conditions of the shock flow in which dust particles can be heated up to the melting point have been revealed in previous work. However, the heating condition is not enough to form the chondrules. We examined, in a framework of the shock-wave heating model, if the shock flow can keep the heated dust particles molten stably. We numerically simulated the heating process in the flow taking into account the evaporation from the dust surface and the ram pressure from the flow onto the particle. We found that the complete vaporization condition does not change significantly, though the evaporation and the latent heat cooling take place during the heating process. We also found that the total gas pressure acting on the particle, which is a summation of the static pressure and the ram pressure, exceeds the vapor pressure of the molten dust particle. Therefore, it is possible to form chondrules, which are once-molten dust particles, in the shock-wave heating model if the dust temperature increases up to the melting point....
Other Link： http://orcid.org/0000-0002-3380-5302
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A Shock-Wave Heating Model for Chondrule Formation: Maximal Size of Chondrules
AAS/Division for Planetary Sciences Meeting Abstracts #33 2001.11
Single Work
Several models of heating events, which formed chondrules, have been proposed to date, though a consensus has not been reached yet. Shock-wave heating model is one of the possible models. In the model, dust particles are heated by the drag heating in the post shock flow. The maximum temperature and the cooling rates of chondrules that are estimated from laboratory experiments can be obtained in the model. Here we show that two other features of chondrules, characteristic size and rotation rates, may also be explained naturally in the framework of the shock-wave heating model. In the model, the sizes of chondrules are bounded by the balance between the surface tension and the ram pressure on a molten droplet. As the size of a molten particle increases, the surface tension descreses. So a large droplet whose radius exceeds a critical value is expected to fragment into small droplets. The acceptable upper bound is around 1 mm, which is consistent with characteristic sizes of chondrules. The upper bound does not depend on the shock flow conditions significantly, i.e., almost all the shocks which heat the dust particles up to the melting point provide similar maximal sizes of chondrules. Moreover, when a chondrule forms through the distribution of a larger droplet, it acquires a spin of the order of 1 x 103 rad s-1. It is also consistent with observations....
Other Link： http://orcid.org/0000-0002-3380-5302
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A shock heating model for chondrule formation in a protoplanetary disk
Iida A, Nakamoto T, Susa H, Nakagawa Y.
Icarus 153 ( 2 ) 430 - 450 2001.10
Joint Work
Chondrule formation due to the shock heating of dust particles with a wide variety of shock properties are examined. We numerically simulate the steady postshock region in a framework of one-dimensional hydrodynamics, taking into account many of the physical and chemical processes that determine the properties of the region, especially nonequilibrium chemical reactions of gas species. We mainly focus on the maximum temperature of dust particles and their net cooling rate in relation to the chondrule formation. We derive the condition of chondrule formation for the shock velocity vs and the preshock density n0. For n0>10 14.5 cm -3, the shock velocity should be in a range 6 km s -1≤ vs≤7 km s -1, while for n0<10 14.5 cm -3, vs should be 6×( n0/10 14.5 cm -3) -1/5 km s -1≤ vs≤7×( n0/10 14.5 cm -3) -1/5 km s -1 for an initial dust particle radius of 0.1 mm. The condition has a small dependence on particle size. We find that the Keplerian velocities and equatorial plane densities around the asteroidal and Jupiter orbital regions of the minimum mass solar nebula model are suitable for chondrule formation. We also find that the gas pressure in the postshock region is much higher than the one in the standard nebula environment. Furthermore, we find that the net cooling rates of 0.1-1-mm-sized dust particles are about 10 2-10 5 K h -1, which are not too far from experimental values, though the melting region is optically thin. Those slow net cooling rates are maintained by drag heating in the cooling phase. These results indicate that the shock heating model can be regarded as a strong candidate for the mechanism of chondrule formation....
Other Link： http://orcid.org/0000-0002-3380-5302
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Criteria for the formation of population III objects in the ultraviolet background radiation
Kitayama T, Susa H, Umemura M, Ikeuchi S.
Monthly Notices of the Royal Astronomical Society 326 ( 4 ) 1353 - 1366 2001.10
Joint Work
We explore possibilities of collapse and star formation in Population III objects exposed to the external ultraviolet background (UVB) radiation. Assuming spherical symmetry, we solve self-consistently radiative transfer of photons, non-equilibrium H2 chemistry and gas hydrodynamics. Although the UVB does suppress the formation of low-mass objects, the negative feedback turns out to be weaker than previously suggested. In particular, the cut-off scale of collapse drops significantly below the virial temperature Tvir~104K at weak UV intensities (J21<~10-2), owing to both self-shielding of the gas and H2 cooling. Clouds above this cut-off tend to contract highly dynamically, further promoting self-shielding and H2 formation. For plausible radiation intensities and spectra, the collapsing gas can cool efficiently to temperatures well below 104K before rotationally supported and the final H2 fraction reaches ~10-3. Our results imply that star formation can take place in low-mass objects collapsing in the UVB. The threshold baryon mass for star formation is ~109Msolar for clouds collapsing at redshifts z<~3, but drops significantly at higher redshifts. In a conventional cold dark matter universe, the latter coincides roughly with that of the 1σ density fluctuations. Objects near and above this threshold can thus constitute `building blocks' of luminous structures, and we discuss their links to dwarf spheroidal/elliptical galaxies and faint blue objects. These results suggest that the UVB can play a key role in regulating the star formation history of the Universe....
DOI： 10.1111/j.1365-2966.2001.04669.x
Other Link： http://orcid.org/0000-0002-3380-5302
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The effects of radiative transfer on the reionization of an inhomogeneous universe
Nakamoto T, Umemura M, Susa H.
Monthly Notices of the Royal Astronomical Society 321 ( 4 ) 593 - 604 2001.3
Joint Work
Assuming simple dynamics for the growth of density fluctuations, we implement six-dimensional (6D) radiative transfer calculations to elucidate the effects of photon propagation during the reionization of an inhomogeneous universe. The ionizing sources are postulated to be AGN-like in this paper. The present simulations reveal that radiative transfer effects are still prominent considerably after the percolation epoch, in which patchy ionized regions connect with each other. In other words, owing to the collective opacity, the Universe does not become perfectly transparent against ionizing radiation even though strongly self-shielded regions disappear. It turns out that the inhomogeneity of the medium enhances the opacity effects and delays the end of reionization. Owing to such radiative transfer effects, the reionization in an inhomogeneous universe proceeds fairly slowly, in contrast to the prompt reionization in a homogeneous universe, and as a result the surface of reionization is not so sharply edged, but highly uneven. As a signature of the uneven surface of reionization, the cosmic IR background (CIB) radiation, which is produced by Lyα photons resulting from radiative recombination, could exhibit strong anisotropies, reflecting the amplitude of density fluctuations at the reionization era. The predicted CIB intensity lies on a level of possible detection by forthcoming IR space telescope facilities....
DOI： 10.1046/j.1365-8711.2001.04008.x
Other Link： http://orcid.org/0000-0002-3380-5302
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3D Radiative Transfer Effects on the Cosmic Reionization
The Physics of Galaxy Formation 2001
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Cosmological Formation of Subgalactic Objects and Radiative Feedback
The Physics of Galaxy Formation 2001
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Cosmic Reionization and Galaxy Formation
The Physics of Galaxy Formation 2001
Single Work
Using 3D radiative transfer calculations on the reionization of an inhomogeneous universe, QSO absorption line systems are simulated and they are compared with observations of Ly$\alpha$ continuum depression at high redshifts. By this comparison, it is found that the metagalactic UV intensity decreases rapidly with $z$ at $z>4$ as $I_{21}=0.5\exp [3(4-z)]$, and the reionization must have taken place between $z=6$ and 10. Based on this time-dependence of UV background intensity, we explore the collapse of pregalactic clouds in the UV background, and find that the self-shielding is prominent above a mass scale as $ M_{BIF}=3.0\times 10^{11} M_\odot [(1+z_c)/5]^{-4.2}(I_{21}/0.5)^{0.6}. $ This mass scale predicts the bifurcation of galactic morphology, and by confrontation with observations it turns out that the bifurcation mass successfully discriminates between elliptical and spiral galaxies....
Other Link： http://orcid.org/0000-0002-3380-5302
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Impacts of Radiative Feedback on the Formation of Primordial Galaxies
The Physics of Galaxy Formation 2001
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UV Background-Induced Bifurcation into Elliptical and Spiral Galaxies
Birth and Evolution of the Universe 2001
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The Physics of Galaxy Formation
The Physics of Galaxy Formation 2001
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Reionization of an Inhomogeneous Universe
Birth and Evolution of the Universe 2001
Single Work
Full radiative transfer in 3D space has been solved to pursue the reionization history in an inhomogeneous universe. It has been shown that the reionization of an inhomogeneous universe is not a prompt event, but a fairly slow process. Also, QSO absorption line systems are simulated with using the results of radiative transfer calculations. Ly$\alpha$ continuum depression implies that the metagalactic UV intensity decreases rapidly with $z$ at $z>5$, and the reionization must have taken place between $z=6$ and 10. Finally, it is stressed that H$\alpha$ forest is a more powerful tool to probe the reionization history and the density fluctuations in the universe at $z>5$....
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Photoionization of a Clumpy Universe
Astrophysics and Space Science Library 2001
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Collapse of low-mass clouds in the presence of a UV radiation field
Susa H, Kitayama T.
Monthly Notices of the Royal Astronomical Society 317 ( 1 ) 175 - 178 2000.9
Joint Work
The collapse of marginally Jeans-unstable primordial gas clouds in the presence of a UV radiation field is discussed. Assuming that the dynamical collapse proceeds approximately in an isothermal self-similar fashion, we investigate the thermal evolution of the collapsing central core until H2 cooling dominates photoheating and the temperature drops to below 104K. Consequently, the mass of the cooled core is evaluated as Mcool=3.6×106Msolar (I21/1)-0.32. This scale depends only on the incident UV intensity, and provides a lower limit to the mass of collapsed objects in the UV radiation field....
DOI： 10.1046/j.1365-8711.2000.03616.x
Other Link： http://orcid.org/0000-0002-3380-5302
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Ultraviolet-background-induced bifurcation of galactic morphology
Susa H, Umemura M.
Monthly Notices of the Royal Astronomical Society 316 ( 2 ) 2000.8
Joint Work
Based upon a novel paradigm of galaxy formation under the influence of an ultraviolet background, the evolutionary bifurcation of pre-galactic clouds is compared with observations of elliptical and spiral galaxies. The theory predicts that the dichotomy between dissipational and dissipationless galaxy formation stems from the degree of self-shielding from the ultraviolet background radiation. This is demonstrated on a bifurcation diagram of collapse epochs versus masses of pre-galactic clouds. Using the observed properties, the collapse epochs and mass are assessed for each type of galaxy. By direct comparison of the theory with observations, it turns out that the theoretical bifurcation branch successfully discriminates between elliptical and spiral galaxies. This suggests that the ultraviolet background radiation could play a profound role in the differentiation of galactic morphology into the Hubble sequence....
DOI： 10.1046/j.1365-8711.2000.03732.x
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation of Primordial Galaxies under Ultraviolet Background Radiation
The Astrophysical Journal 2000.7
Single Work
The pancake collapse of pregalactic clouds under UV background radiation is explored with a one-dimensional sheet model. Here, attention is concentrated on elucidating the basic physics on the thermal evolution of pregalactic clouds exposed to diffuse UV radiation. So, we treat accurately the radiation transfer for the ionizing photons, with solving chemical reactions regarding hydrogen molecules as well as atoms. The self-shielding against UV radiation by H2 Lyman-Werner bands, which regulates the photodissociation of hydrogen molecules, is also taken into account. As a result, it is found that when the UV background radiation is at a level of 10-22(ν/νL)-1 ergs s-1 cm-2 Hz-1 sr-1, the cloud evolution bifurcates with a critical mass as MSB=2.2×1011 Msolar[(1+zc)/5]-4.2, where zc is the final collapse epoch. A cloud more massive than MSB cools below 5×103 K owing to H2 line emission at the pancake collapse and would undergo the initial starburst. The pancake possibly evolves into a virialized system in a dissipationless fashion. Consequently, this leads to the dissipationless galaxy formation at 3<~zc<~10. A cloud less massive than MSB cannot cool by H2 emission shortly after the pancake collapse but could cool in the course of shrinking to the rotation barrier. This is likely to lead to the dissipational galaxy formation at relatively low redshifts as 0<~zc<~4. The present results provide a solid physical mechanism that controls the star formation efficiency in the pregalactic clouds. In the context of a standard CDM cosmology, MSB lies between 1 σ and 2 σ density fluctuations....
DOI： 10.1086/309039
Other Link： http://orcid.org/0000-0002-3380-5302
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Radiation-hydrodynamical collapse of pre-galactic clouds in the ultraviolet background
Kitayama T, Tajiri Y, Umemura M, Susa H, Ikeuchi S.
Monthly Notices of the Royal Astronomical Society 315 ( 1 ) 2000.6
Joint Work
To explain the effects of the ultraviolet (UV) background radiation on the collapse of pre-galactic clouds, we implement a radiation-hydrodynamical calculation, combining one-dimensional spherical hydrodynamics with an accurate treatment of the radiative transfer of ionizing photons. Both absorption and scattering of UV photons are explicitly taken into account. It turns out that a gas cloud contracting within the dark matter potential does not settle into hydrostatic equilibrium, but undergoes run-away collapse even under the presence of the external UV field. The cloud centre is shown to become self-shielded against ionizing photons by radiative transfer effects before shrinking to the rotation barrier. Based on our simulation results, we further discuss the possibility of H2 cooling and subsequent star formation in a run-away collapsing core. The present results are closely relevant to the survival of subgalactic Population III objects as well as to metal injection into intergalactic space....
DOI： 10.1046/j.1365-8711.2000.03589.x
Other Link： http://orcid.org/0000-0002-3380-5302
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Photoionization and dynamical evolution of pregalactic clouds
Nuclear Physics B Proceedings Supplements 2000.1
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Formation of primordial galaxies under ultraviolet background radiation
Susa H, Umemura M.
Astrophysical Journal 537 ( 2 PART 1 ) 578 - 588 2000
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On the Formation of Primordial Stars and Pop III Luminous Objects
The First Stars 2000
Single Work
We investigate the formation site of the primordial stars, the formation process of them and the feedback on the host clouds. First, the `cooling diagram' for cosmologicallow mass objects is shown. We assess the cooling rate taking into account the contribution of H2, which is not in chemical equilibrium generally. Next, we show the results of the hydrodynamical simulation for primordial star formation, which includes radiative transfer and the chemistry of H2. Moreover, the feed back by the first generation massive stars is considered. The main feedback consists of two different processes, UV radiation from the stars and energy input by SNe. Finally, we investigate the formation condition of luminous objects and the mass of the Pop III luminous objects is estimated to be several times 107 M_☉....
DOI： 10.1007/10719504_49
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation of Primordial Stars and Evolution of Primordial Gas Cloud
Star Formation 1999 1999.12
Single Work
We investigate the evolution of cosmological low mass (low virial temperature) clouds and the formation of the first stars. We also estimate the feedback by the first generation stars to the host clouds. First, the `cooling diagram' for low mass objects is shown. We assess the cooling rate taking into account the contribution of Hydrogen molecules, which is not in chemical equilibrium generally, with a simple argument of time scales. The reaction rates and the cooling rate are taken from the recent results by Galli and Palla (1998). Next, we show the simulation of the dynamical collapse of metal free protostellar clouds and formation of protostars. We performed hydrodynamical calculations for spherically symmetric clouds taking account of radiative transfer of the molecular hydrogen lines and the continuum, as well as of chemistry of the molecular hydrogen. The collapse is non-homologous and proceed almost self-similarly like Larson-Penston similarity solution. The effective `adiabatic index' at the center is about 1.1 almost throughout the collapse. The mass accretion rate is estimated to be very large and the massive star formation is expected. Finally, we investigate the feed back by the first generation massive stars. The main feedback consists of two different processes, UV radiation from the stars and energy input by SNe. The feedback from the formed stars on their own host cloud is very serious....
Other Link： http://orcid.org/0000-0002-3380-5302
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Origin of Hubble Sequence --- Bifurcation into Spiral and Elliptical Galaxies ---
Star Formation 1999 1999.12
Single Work
Evolutionary bifurcation of cosmological density perturbation is discussed according to the galaxy formation theory proposed by Susa & Umemura (1999). We plot the observed elliptical and spiral galaxies on the cosmological initial condition plane (collapse epoch v.s. mass), with the help of virial theorem. On the initial condition plane, the boundary between the elliptical and spiral galaxies coincide very well with the predicted line by the theory....
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation and Disruption of Cosmological Low-Mass Objects
The Astrophysical Journal 1999.10
Single Work
We investigate the evolution of cosmological low-mass (low virial temperature) objects and the formation of the first luminous objects. First, the ``cooling diagram'' for low-mass objects is shown. We assess the cooling rate taking into account the contribution of H2, which is not in chemical equilibrium generally, with a simple argument of timescales. The reaction rates and the cooling rate of H2 are taken from the recent results by Galli & Palla. Using this cooling diagram, we also estimate the formation condition of luminous objects taking into account the supernova (SN) disruption of virialized clouds. We find that the mass of the first luminous object is several times 107 Msolar because smaller objects may be disrupted by the SNe before they become luminous. Metal pollution of low-mass (Lyα) clouds is also discussed. The resultant metallicity of the clouds is Z/Zsolar~10-3....
DOI： 10.1086/312277
Other Link： http://orcid.org/0000-0002-3380-5302
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3D Radiative Transfer Calculations on the Cosmic ReionizationCD
Numerical Astrophysics 1999
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Fragmentation of the Primordial Gas Clouds
Cosmological Parameters and the Evolution of the Universe 1999
Single Work
We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the timescale of collapse with that of fragmentation, we obtain the typical mass of a fragment both numerically and analytically. We show that the estimated mass gives the minimum mass of a fragment which is formed from the primordial gas cloud and is essentially determined by the Chandrasekhar mass....
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation and disruption of cosmological low-mass objects
Nishi R, Susa H.
Astrophysical Journal 523 ( 2 PART 2 ) 1999
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Evolution of Primordial Protostellar Clouds
Cosmological Parameters and the Evolution of the Universe 1999
Single Work
We present results of the hydrodynamical calculation of collapsing primordial proto-stellar clouds. In these calculations, the radiative transfer of molecular hydrogen lines is considered. The molecular hydrogen line cooling is so effective, even when a cold and optically thick envelope is present, that the cloud can collapse at almost free-fall rate. This phenomenon is peculiar to line cooling. A line is not infinitely sharp, but has some width mainly due to thermal Doppler shift. This means that even if the cloud becomes optically thick at the line centre,it is still optically thin at the line edge. Therefore photons can still escape from the clouds....
Other Link： http://orcid.org/0000-0002-3380-5302
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Photoionization and Dynamical Evolution of Mini-PancakesCD
Numerical Astrophysics 1999
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Photoionization and Dynamical Evolution of Pregalactic Clouds
19th Texas Symposium on Relativistic Astrophysics and Cosmology 1998.12
Single Work
We investigate thermal and hydrodynamical evolution of collapsing pancake in the era of galaxy formation. We solve detailed chemical rate equations for six species, e, H, H^+, H^-, H^+_2 and H_2. We compute photoionization and photodissociation rates by the UV radiation field. We integrate the radiation transfer equation for the ionizing photons (h nu >= 13.6eV) strictly. When the pancake is irradiated by the UV background radiation field (J21 = 0.1, J_nu propto nu^{-alpha}, alpha = 1), we find a critical thickness with a given initial averaged density bar{n}_ini. It is lambda_cool = 1.1 Mpc (bar{n}_ini/1.0 times 10^{-4} cm^{-3})^{-0.8} that determines whether the sheet will cool or not. If the initial thickness of the sheet is larger than lambda_cool, the sheet collapses and cool below 5 times 10^3K by H_2 rovibrational line emission. Otherwise, the temperature of the sheet is high (T > 10^4 K) throughout the evolution, even if it collapse....
Other Link： http://orcid.org/0000-0002-3380-5302
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Thermal and dynamical evolution of primordial gas clouds: On the formation of first luminous objects
Nishi R, Susa H, Uehara H, Yamada M, Omukai K.
Progress of Theoretical Physics 100 ( 5 ) 881 - 903 1998.11
Joint Work
We investigate the thermal and dynamical evolution of primordial gas clouds in the universe after decoupling. Comparing the time scale of dynamical evolution with that of fragmentation, we can estimate the typical fragmentation scale. We propose the following scenario of the formation process of first luminous objects consisting of large number stars. First, by pancake collapse of the overdensity regions in the expanding universe or collision between clouds in potential wells, quasi-plane shocks form. If the shock-heated temperature is higher than about 104 K, the post-shock gas cools down to several hundred K by H2 line cooling, and the shock-compressed layer fragments into filamentary clouds. The filamentary cloud collapses dynamically once more and fragments into cloud cores. Finally, a primordial star forms in a cloud core. We show that the minimum mass of the first star is essentially determined by the Chandrasekhar mass. Also, we investigate the dynamical collapse of cloud cores by numerical simulation and show that the evolution paths of the central regions of the cores depend only very weakly on the total core mass. After mass accretion, a massive star may be formed in a core, since the estimated mass accretion rate is very large. In such a case, it may be possible for many massive stars form almost simultaneously. Then the clouds can be luminous objects. On the other hand, if the shock-heated temperature is lower, effective star formation is delayed significantly....
DOI： 10.1143/PTP.100.881
Other Link： http://orcid.org/0000-0002-3380-5302
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Evolution of primordial protostellar clouds quasi-static analysis
Omukai K, Nishi R, Uehara H, Susa H.
Progress of Theoretical Physics 99 ( 5 ) 747 - 761 1998.5
Joint Work
The contraction processes of metal-free molecular clouds of starlike mass (or cloud cores) are investigated. We calculate radiative transfer of the H2 lines and examine quasi-static contraction with radiative cooling. Comparing two time-scales, the free-fall time tff and the time-scale of quasi-static contraction tqsc (~ tcool, the cooling time) of these cores, we find that the ratio of the two time-scales tff/tqsc, i.e., the efficiency of cooling, becomes larger with contraction even under the existence of cold and opaque envelopes. In particular, for fragments of primordial filamentary clouds, for which tff ~ tqsc at the fragmentation epoch, they collapse dynamically in the free-fall time-scale. This efficiency of cooling is unique to line cooling....
DOI： 10.1143/PTP.99.747
Other Link： http://orcid.org/0000-0002-3380-5302
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On the mass of first generation stars and formation of MACHOS
Memorie della Societa Astronomica Italiana 1998
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Thermal evolution in post-shock layer
Memorie della Societa Astronomica Italiana 1998
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Thermal Evolution of Primordial Gas Clouds - A clue to Galaxy Formation -
Hajime Susa
1997.3
Single Work
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Fragmentation of the primordial gas clouds and the lower limit on the mass of the first stars
Uehara H, Susa H, Nishi R, Yamada M, Nakamura T.
Astrophysical Journal 473 ( 2 PART II ) 1996.12
Joint Work
We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the timescale of collapse with that of fragmentation, we obtain the typical mass of a fragment both numerically and analytically. We show that the estimated mass gives the minimum mass of a fragment that is formed from the primordial gas cloud and essentially determined by the Chandrasekhar mass....
DOI： 10.1086/310409
Other Link： http://orcid.org/0000-0002-3380-5302
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The fragment mass scale of the primordial gas clouds. I: Non-spherical pressure-free collapse
Susa H, Uehara H, Nishi R.
Progress of Theoretical Physics 96 ( 6 ) 1073 - 1086 1996.12
Joint Work
We investigate the thermal evolution of non-spherical primordial gas clouds of mass M = 106Modot. We study two cases: 1) prolate and oblate clouds without angular momentum, 2) initially spherical, oblate and prolate clouds with angular momentum. In the spherical case, the magnitude of the angular momentum is the key quantity which determines the fragment mass. The fragment mass is found to be M >~ 90Modot for 10-2 <~ λ <~ 1, where λ is the cosmological spin parameter. For an oblate shape of initial gas clouds, the angular momentum is almost never important. In the case of prolate clouds with some angular momentum, collapse proceeds in the same way as in the spherical case, and a bounce occurs to form a disk due to the angular momentum. In any case, the estimated `fragment mass' must be almost above 90Modot for primordial gas clouds with typical angular momentum and typical oblateness or prolateness....
DOI： 10.1143/PTP.96.1073
Other Link： http://orcid.org/0000-0002-3380-5302
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Probability Distribution of Primordial Angular Momentum and Formation of Massive Black Holes
Progress of Theoretical Physics 1994.11
Single Work
We consider the joint probability distribution function for the mass contrast and angular momentum of over-density regions on the protogalactic scale and investigate the formation of massive black holes at redshift z >~ 10. We estimate the growth rate of the angular momentum by the linear perturbation theory and the decay rate by the Compton drag and apply the Press-Schechter theory to obtain the formation rate of massive black holes, assuming the full reionization of the universe at z = zion >> 10. We find the correlation between the mass contrast and angular momentum vanishes in the linear theory. However, application of the Press-Schechter theory introduces a correlation between the mass contrast and angular momentum of bound objects. Using thus obtained probability distribution, we calculate the mass fraction of black holes with M ~ 106 - 108 Modot in the universe. We find that it crucially depends on the reionization epoch zion. Specifically, for the standard CDM power spectrum with the COBE normalization, the condition zion >~ 500 must be satisfied to reproduce the observed number density of QSOs....
DOI： 10.1143/ptp/92.5.961
Other Link： http://orcid.org/0000-0002-3380-5302
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A numerical study of galaxy formation and the large scale structure of the universe.
Evolution of the Universe and its Observational Quest 1994
Single Work
The authors investigate the structure formation of the universe with hydrodynamical simulations. The galaxy formation is treated as cells which satisfy a certain condition. They consider the effects of galactic bursts on forming galaxies and the structure of the IGM, the angular correlation function of galaxies, and the luminosity functions of X-ray clusters....
Other Link： http://orcid.org/0000-0002-3380-5302