

The Ly
forest represents the optically thin (at the Lyman edge)
component of Quasar Absorption Systems (QAS), a collection of
absorption features in QSO 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 Ly
forest have been performed recently [28,
47,
64], and all have been able to fit the observations remarkably
well, including the column density and Doppler width
distributions, the size of absorbers [26], 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 Ly
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, including fluctuations in underdense
regions, spheroidal minihalos, and filaments extending over
scales of a few megaparsecs (figure
4). However, it is not yet clear what effect different
cosmological models have on these systems.
Figure 4:
Distribution of the gas density at redshift
z
=3 from a numerical hydrodynamics simulation of the Ly
forest. The simulation adopted a CDM spectrum of primordial
density fluctuations, normalized to the 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 cosmic density (characteristic of typical
filamentary structures) and are color coded to the gas
temperature (dark blue =
K, light blue =
K). The higher density contours trac e 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.
Notice that there is fine structure everywhere. 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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Physical and Relativistic Numerical Cosmology
Peter Anninos
http://www.livingreviews.org/lrr-1998-2
© Max-Planck-Gesellschaft. ISSN 1433-8351
Problems/Comments to
livrev@aei-potsdam.mpg.de
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