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NED Abstract
Copyright by American Astronomical Society.
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2000ApJ...543..552S
Surveys for z > 3 Damped Ly{alpha} Absorption Systems: The Evolution of
Neutral Gas
Lisa J. StorrieLombardi
SIRTF Science Center, California Institute of Technology, MS 10022,
Pasadena, CA 91125; lisa@ipac.caltech.edu
and
Arthur M. Wolfe
Department of Physics, and Center for Astrophysics and Space Sciences,
University of California at San Diego, La Jolla, CA 920930424
Received 2000 March 2; accepted 2000 June 12
ABSTRACT
We have completed spectroscopic observations using LRIS on the Keck 1
telescope of 30 very high redshift quasars, 11 selected for the presence of
damped Ly{alpha} absorption systems and 19 with redshifts z > 3.5 not
previously surveyed for absorption systems. We have surveyed an additional
10 QSOs with the Lick 120" and the AngloAustralian Telescope. We have
combined these with previous data, resulting in a statistical sample of 646
QSOs and 85 damped Ly{alpha} absorbers with column densities N_HI_ >= 2 x
10^20^ atoms cm^2^ covering the redshift range 0.008 < z < 4.694. Four
main features of how the neutral gas in the universe evolves with redshift
are evident from these data.
1. For the first time, we determine a statistically significant
steepening in the column density distribution function at redshifts z > 4.0
(greater than 99.7% confidence). The steepening of the distribution
function is due to both fewer very high column density absorbers (N_HI_ >=
10^21^ atoms cm^2^) and more lower column density systems (N_HI_ = 24 x
10^20^ atoms cm^2^).
2. The frequency of very high column density absorbers (N_HI_ >= 10^21^
atoms cm^2^) reaches a peak in the redshift range 1.5 < z < 4, when the
universe is 10%30% of its present age. Although the sample size is still
small, the peak epoch appears to be 3.0 <= z <= 3.5. The highest column
density absorbers disappear rapidly toward higher redshifts in the range z
= 3.5 > 4.7 and lower redshifts z = 3.0 > 0. None with column densities
log N_HI_ > 21 have yet been detected at z > 4, although we have increased
the redshift path surveyed by ~60%.
3. With our current data set, the comoving mass density of neutral gas,
{OMEGA}_g_, appears to peak at 3.0 < z < 3.5, but the uncertainties are
still too large to determine the precise shape of {OMEGA}_g_. The
statistics are consistent with a constant value of {OMEGA}_g_ for 2 < z <
4. There is still tentative evidence for a dropoff at z > 4, as
indicated by earlier data sets. If we define R_g*_ = {OMEGA}_g_/
{OMEGA}_*_, where R_g*_ is the ratio of the peak value of {OMEGA}_g_ to
{OMEGA}_*_, the mass density in galaxies in the local universe, we find
values of R_g*_ = 0.250.5 at z ~ 3, depending on the cosmology. For an
{OMEGA} = 1 universe with a zero cosmological constant, R_g*_ ~ 0.5. For an
{OMEGA} = 1 universe with a positive cosmological constant
({OMEGA}_{LAMBDA}_ = 0.7, {OMEGA}_M_ = 0.3), we find R_g*_ ~ 0.25. For a
universe with {OMEGA}_{LAMBDA}_ = 0 and {OMEGA}_M_ = 0.3, we find R_g*_ ~
0.3.
4. {OMEGA}_g_ decreases with redshift for the interval z = 3.5 > 0.008
for our data set, but we briefly discuss new results from Rao & Turnshek
for z < 1.5 that suggest that {OMEGA}_g_ (z < 1.5) may be higher than
previously determined.
To make the data in our statistical sample more readily available for
comparison with scenarios from various cosmological models, we provide
tables that include all 646 QSOs from our new survey and previously
published surveys. They list the minimum and maximum redshift defining the
redshift path along each line of sight, the QSO emission redshift, and when
an absorber is detected, the absorption redshift and measured H I column
density.
Subject headings: galaxies: evolutionintergalactic mediumquasars:
absorption lines
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