

The Lyman-alpha forest represents the optically thin (at the
Lyman edge) component of Quasar Absorption Systems (QAS), a
collection of absorption features in quasar spectra extending
back to high redshifts. QAS are effective probes of the matter
distribution and the physical state of the Universe at early
epochs when structures such as galaxies are still forming and
evolving. Although stringent observational constraints have been
placed on competing cosmological models at large scales by the
COBE satellite and over the smaller scales of our local Universe
by observations of galaxies and clusters, there remains
sufficient flexibility in the cosmological parameters that no
single model has been established conclusively. The relative lack
of constraining observational data at the intermediate to high
redshifts (0 <
z
< 5), where differences between competing cosmological models
are more pronounced, suggests that QAS can potentially yield
valuable and discriminating observational data.
Several combined N-body and hydrodynamic numerical simulations
of the Lyman forest have been performed recently [26,
42,
60], and all have been able to fit the observations remarkably
well, including the column density and Doppler width
distributions, the size of absorbers [24], and the line number evolution. Despite the fact that the
cosmological models and parameters are different in each case,
the simulations give similar results, provided that the proper
ionization bias is used (
, where
is the baryonic density parameter,
h
is the Hubble parameter and
is the photoionization rate at the hydrogen Lyman edge). A
theoretical paradigm has thus emerged from these calculations in
which Lyman-alpha absorption lines originate from the relatively
small scale structure in pregalactic or intergalactic gas through
the bottom-up hierarchical formation picture in CDM-like
universes. The absorption features originate in structures
exhibiting a variety of morphologies commonly found in numerical
simulations (see Figure
5), including fluctuations in underdense regions, spheroidal
minihalos, and filaments extending over scales of a few
megaparsecs.
Figure 5:
Distribution of the gas density at redshift
z
=3 from a numerical hydrodynamics simulation of the Lyman-alpha
forest with a CDM spectrum normalized to second year COBE
observations, a Hubble parameter of
h
=0.5, a comoving box size of 9.6 Mpc, and baryonic density of
composed of 76% hydrogen and 24% helium. The region shown is 2.4
Mpc (proper) on a side. The isosurfaces represent baryons at ten
times the mean density and are color coded to the gas temperature
(dark blue =
K, light blue =
K). The higher density contours trace out isolated spherical
structures typically found at the intersections of the filaments.
A single random slice through the cube is also shown, with the
baryonic overdensity represented by a rainbow-like color map
changing from black (minimum) to red (maximum). The He
mass fraction is shown with a wire mesh in this same slice. To
emphasize fine structure in the minivoids, the mass fraction in
the overdense regions has been rescaled by the gas overdensity
wherever it exceeds unity.


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Computational Cosmology: from the Early Universe to the
Large Scale Structure
Peter Anninos
http://www.livingreviews.org/lrr-1998-9
© Max-Planck-Gesellschaft. ISSN 1433-8351
Problems/Comments to
livrev@aei-potsdam.mpg.de
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