Japanese Version
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"Three-Dimensional Hydrodynamical Calculations on the Fragmentation of
Pancakes and Galaxy Formation"
M. Umemura,
Astrophysical Journal, 1993, 406, 361-382
The fragmentation of pancakes is numerically investigated
by a 3D smoothed particle hydrodynamics (SPH) combined with
a usual N-body scheme, where the ionization equilibria are solved
and all kinds of the cooling and heating processes
are explicitly included for primordial gas.
We consider dark matter-dominated pancakes as
well as baryon-dominated ones,
in a range of pancake mass of 1013-15Msun.
As a result, we find a three-step
fragmentation scenario:
First, the fragmentation is triggered by the
filamentary mode instability in a central cooled dense layer.
Secondly, they are torn to numerous lumps,
forming chain-like structures.
Finally, the lumps flow toward the outer edge and
a central hole develops. Then the lumps collide with each other and
merge into larger ones.
The final stage of hole-making may lead to a kind of
dynamical anti-biasing in the sense that galaxies
eventually form far away from peaks of initial density
fluctuations.
The fragmentation process turns out to be strongly dependent
upon imposed small-scale fluctuations. Unless there are some small-scale
seed fluctuations imposed, the pancakes will not fragment.
Therefore, a hot dark matter (HDM)-dominated universe
doesn't seem preferable as a complete model of galaxy formation.
It is found that just a small fluctuation amplitude is required
for fragmentation at smaller scales
than the HDM free-streaming damping mass.
A two-component dark matter-dominated universe
turns out to be successful from a viewpoint of galaxy formation.
- "Early Cosmic Formation of Massive Black Holes"
M. Umemura, A. Loeb, and E. L. Turner,
Astrophysical Journal, 1993, 419, 459-468
The evolution of nonlinear density fluctuations around the Jeans mass
shortly after cosmological recombination is analyzed using a 3D
hydrodynamics/dark-matter code. The Cosmic Background Radiation (CBR)
exerts Compton friction on free electrons in resistance to their
peculiar velocities.
The baryonic dynamics therefore depends strongly
on the gas ionization history.
For a variety of scenarios for the local re-ionization
and in systems with or without non-baryonic components,
the baryons lose angular momentum
efficiently and collapse to form a compact optically-thick object
which would probably quickly evolve into a massive black hole.
- "Reionization of the Universe due to Early-Formed Massive Black Holes"
S. Sasaki and M. Umemura,
Astrophysical Journal, 1996, 462, 104-109
Based on a recently proposed scenario
on the early formation of massive black holes,
the possibility that the universe is reionized due to the UV radiation from
the black holes is investigated.
We consider a two-step scenario for the formation
of an active compact nucleus: First, an angular momentum
supported disk shrinks into a compact disk due to the
Compton drag with the cosmic background radiation.
Second, the disk is adiabatically
heated and then the turbulent viscosity
works to convert the gravitational energy into powerful radiation.
Within this context, it is found that the overall reionization of
the universe is possible by high redshift black hole systems.
The reionization epoch depends upon the power-law index
of the density fluctuations at the recombination epoch.
If the spectral index
is in the range of n > -1.7,
the universe is reionized overall by z=150, having the
Gunn-Peterson optical depth
below the 2 sigma upper limits inferred
from the recent observations of QSOs at 2.6 < z < 4.2.
The UV flux from
the early-formed massive black holes may provide additional sources to
the UV background intensity to satisfy the "proximity effect" of
Lyman alpha absorption lines in QSO spectra. Also,
the Compton y-parameter of the CBR
is considerably below the observed upper limits from the COBE results.
If n < -1.5, the results are not in conflict
with the observed soft X-ray background. The predicted
He Gunn-Peterson effect and
mass function of relic black holes are also presented.
- "Very Strong Microlensing of Distant Luminous Stars by Relic
Massive Black Holes"
E. L. Turner and M. Umemura,
Astrophysical Journal, 1997, 483, 603-607
We examine the possibility that a population of relic massive black holes,
perhaps constituting an important component of the dark matter, might be
indirectly detected via their occasional very strong gravitational lensing
of individual luminous stars in distant external galaxies. For plausible, and
in some respects conservative, values of the relevant physical parameters,
we show that such events might be detected either in wide area surveys reaching
routine CCD magnitude limits (such as the Sloan Digital Sky Survey)
or in small field, very deep images (such as the HDF). Thus, it would
be a challenging
but not impossible task to detect or place limits on a cosmic
population of relic massive black holes.
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"Photoionization of a Clumpy Universe"
T. Nakamoto, H. Susa, and M. Umemura,
International Symposium on Supercomputing,
New Horizon of Computational Science, 1998, in press
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"3D Radiative Transfer Calculations on the Cosmic Reionization"
M. Umemura, T. Nakamoto, and H. Susa,
Numerical Astrophysics 1998, 1998, in press
[
PostScript(164kb)]
[Slide Show of Poster: click here ]
The reionization history in a clumpy universe is pursued
by solving 3D radiative transfer equation for the permeation
of diffuse UV photons. It is found that the the self-shielding
is prominent at the reionization epoch for local high density
peaks even if they are of linear density contrast. In other words,
the cosmic reionization proceeds with exhibiting a very patchy pattern,
and ionized regions are quickly percolated at a certain epoch.
Discussed are the degree of self-shielding or shadowing and near
IR background anisotropy which is generated by Ly alpha photons
at the reionization epoch.
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"The fragment mass scale of the primordial
gas clouds I. non-spherical pressure-free collapse"
Hajime Susa ,Hideya
Uehara, and Ryoichi Nishi, Progress of Theoretical Physics Volume 96, Number
6, pp.1073- 1086 December 1996
- "Fragmentation of the Primordial Gas Clouds
and the Lower Limit of the Mass of the First Stars"
Hideya Uehara, Hajime Susa, Ryoichi Nishi, Masako Yamada,
and Takashi Nakamura, Astrophysical
Journal, Volume 473, L95-L98, 1996 December 20
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"The Thermal Evolution of the Post Shock
Layer in Primordial Gas Clouds"
Hajime Susa, Hideya Uehara, Ryoichi Nishi and Masako Yamada,
in preperation
- "The thermal evolution of primordial gas
clouds: A clue to galaxy formation"
Hajime Susa, Thesis in Kyoto University, Japan
