## 2. Ly samples and problemsIn PF, we looked at two popular data sets of
Ly forests. The first compiled by Lu, Wolfe and
Turnshek (1991, hereafter LWT) contains 950
lines from the spectra of 38 QSO that exhibit neither broad absorption
lines nor metal line systems. The second set is from Bechtold (1994,
hereafter JB), which contains 2800 lines from 78
QSO's spectra, in which 34 high red-shift QSOs were observed at
moderate resolution. In this paper, we augment those data sets with
two observations using the Keck telescope: 1) Hu et al. (1995,
hereafter HKCSR) observed 4 QSO's with a total of 1056 lines and
column density in the range cm
It is well known that the number density of the
Ly absorption lines increases with red-shift. The
number density of lines with rest equivalent width where is the number density extrapolated to zero red-shift, and the index of evolution. LWT finds that and for lines with Å. KT finds while JB finds that for Å and for Å. Like other Ly forest data, these data sets
have failed to reveal structures in their distribution on scales
km/s when subjected to a two point correlation
analysis (Hu et al. 1995.) Using the discrete wavelet transform
spectrum estimator, the Fourier power spectrum of the 1-dimensional
(1-D) spatial distribution of these data is found to be almost flat on
scales from 2 to about 100 h These results indicate that 2nd order statistical techniques, i.e. the two-point correlation function and the power spectrum, are not even qualitatively sufficient to describe the clustering features of these samples. Higher order measures are not a correction to lower order descriptions, but crucial in describing the Ly forest traced matter field. This conclusion is strengthened by studying simulated samples. Typically, simulated density fields for pre-collapsed clouds are generated as perturbations with a linear or linear log-normal spectrum given by models such as the cold dark matter model (SCDM), the cold plus hot dark matter model (CHDM), and the low density flat cold dark matter model (LCDM). The baryonic matter distribution is then produced by assuming that the baryonic matter traces the dark matter distribution on scales larger than the Jeans length of the baryonic gas, but is smooth over structures on scales less than the Jeans length. The simulated Ly absorption spectrum can be calculated as the absorption of neutral hydrogen in the baryonic gas. The effects of the observational instrumental point-spread-function, along with Poisson and background noises can also be simulated properly (Bi, Ge & Fang 1995, hereafter BGF; Bi & Davidson 1997). Within a reasonable set of parameters, the simulated samples are
found to fit with observational measurements such as the number
density of Ly clouds, the distribution of
equivalent widths, the red-shift-dependence of the width distributions
and clustering. Regardless the details of the simulation, these
samples should contain structures because the effects of gravitational
collapse have been considered, and baryonic matter does not distribute
uniformly random, but traces the structure of dark matter. However, as
is the case with observations, the simulated samples show no power in
their two-point correlations (see Fig. 11 of BGF), and their 1-D
spectra are rather flat on scales less than 100 h © European Southern Observatory (ESO) 1998 Online publication: November 9, 1998 |