Many complex multi-component numerical
simulations have been performed of the Lyman forest, which
include the effects of dark matter (N-body), baryons
(hydrodynamics), chemical composition (reactive networks), and
microphysical response (radiative cooling and heating). See, for
example,
[67, 118, 166], which represent some of the earliest comprehensive simulations.
For the most part, all these calculations have been able to fit
the observations reasonably well, including the column density
and Doppler width distributions, the size of absorbers
[62], and the line number evolution. Despite the fact that the
cosmological models and parameters are different in each case,
the simulations give roughly similar results provided that the
proper ionization bias is used,
, where
is the baryonic density parameter,
is the Hubble parameter and
is the photoionization rate at the hydrogen Lyman edge.
(However, see
[50]
for a discussion of the sensitivity of statistical properties on
numerical resolution.) A theoretical paradigm has thus emerged
from these calculations in which Ly
absorption lines originate from the relatively smaller 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
11
), including fluctuations in underdense regions, spheroidal
minihalos, and filaments extending over scales of a few Mpc.
Meiksin et al.
[117]
followed up with more detailed comparisons of Ly
systems in several cosmologies with observed high resolution QSO
spectra. Although all models are consistent with previous studies
in that they give reasonably good statistical agreement with
observed Ly
properties, under closer scrutiny none of the numerical models
they considered passed all the tests, which included spectral
flux, wavelet decomposed amplitude, and absorption line profile
distributions. They suggest that comparisons might be improved,
particularly in optically thin systems, by more energy injection
into the IGM from late
reionization or supernovae-driven winds, or by a larger baryon
fraction.
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