24 research outputs found
HI-selected Galaxies as a probe of Quasar Absorption Systems
We investigate the properties of HI-rich galaxies detected in blind radio
surveys within the hierarchical structure formation scenario using a
semi-analytic model of galaxy formation. By drawing a detailed comparison
between the properties of HI-selected galaxies and HI absorption systems, we
argue a link between the local galaxy population and quasar absorption systems,
particularly for Damped Ly-alpha absorption (DLA) systems and sub-DLA systems.
First, we evaluate how many HI-selected galaxies exhibit HI column densities as
high as those of DLA systems. We find that HI-selected galaxies with HI masses
M(HI) > 10^8 solar masses have gaseous disks that produce HI column densities
comparable to those of DLA systems. We conclude that DLA galaxies where the HI
column densities are as high as those of DLA systems, contribute significantly
to the population of HI-selected galaxies at M(HI) > 10^8 solar masses. Second,
we find that star formation rates (SFRs) correlate tightly with HI masses
rather than B- (and J-) band luminosities.
In the low-mass range M(HI) < 10^8 solar masses, sub-DLA galaxies replace DLA
galaxies as the dominant population. The number fraction of sub-DLA galaxies
relative to galaxies reaches 40%-60% at HI masses 10^8 solar masses and 30%-80%
at 10^7 solar masses. The HI-selected galaxies at 10^7 solar masses are a
strong probe of sub-DLA systems that place stringent constraints on galaxy
formation and evolution.Comment: 25 pages, 13 figures, Accepted for publication in the Astrophysical
Journa
Evolution of Lyman α clouds at low redshifts
We examine the evolution of an intergalactic gas cloud confined by the gravity of cold dark matter in the minihalo model. Assuming that the evolution of the diffuse ultraviolet (UV) flux follows a power law function of the form (1 + z)alphai with a change in exponent at redshift zc and a constant comoving number density of gas clouds, we can reproduce the evolution of the number of clouds per unit redshift at both high and low redshifts with alphaL = 2 for z less than or equal to zc and alphah = -4 (or -3) for z greater than zc with zc = 1.9 (or 1.8). Such a small value of alphaL implies much more UV flux at low redshifts than is expected from the luminosity function of observed quasars. Moreover, we find that the required intensity of this UV flux is consistent with observations of the Gunn-Peterson effect and of the continuum depression
Evolution of the intergalactic medium and UV background radiation
We discuss the evolutions of the intensity and spectrum of the UV background radiation (UVB) in flat and open cold dark-matter universes. We consider two kinds of UV sources: one is QSOs, the other is collapsing objects, which liberate their binding energy radiatively. In the latter sources, we include the line emission corresponding to collisional excitation cooling of HeII as well as free-free emission. We find that UV photons emitted from collapsing objects contribute to UVB as much as those from QSOs, and that the UVB spectrum becomes softer than that in the case in which only QSOs are taken into account. We also calculate the Gunn-Peterson optical depths assuming the ionization equilibrium, and compare them with observations
Damped Lyman alpha Absorbing Galaxies At Low Redshifts z<1 From Hierarchical Galaxy Formation Models
We investigate Damped Ly-alpha absorbing galaxies (DLA galaxies) at low
redshifts z<1 in the hierarchical structure formation scenario to clarify the
nature of DLA galaxies because observational data of such galaxies mainly at
low redshifts are currently available. We find that our model well reproduces
distributions of fundamental properties of DLA galaxies such as luminosities,
column densities, impact parameters obtained by optical and near-infrared
imagings. Our results suggest that DLA systems primarily consist of low
luminosity galaxies with small impact parameters (typical radius about 3 kpc,
surface brightness from 22 to 27 mag arcsec^{-2}) similar to low surface
brightness (LSB) galaxies. In addition, we investigate selection biases arising
from the faintness and from the masking effect which prevents us from
identifying a DLA galaxy hidden or contaminated by a point spread function of a
background quasar. We find that the latter affects the distributions of DLA
properties more seriously rather than the former, and that the observational
data are well reproduced only when taking into account the masking effect. The
missing rate of DLA galaxies by the masking effect attains 60-90 % in the
sample at redshift 0<z<1 when an angular size limit is as small as 1 arcsec.
Furthermore we find a tight correlation between HI mass and cross section of
DLA galaxies, and also find that HI-rich galaxies with M(HI) \sim 10^{9} M_sun
dominate DLA systems. These features are entirely consistent with those from
the Arecibo Dual-Beam Survey which is a blind 21 cm survey. Finally we discuss
star formation rates, and find that they are typically about 10^{-2} M_sun
yr^{-1} as low as those in LSB galaxies.Comment: 21 pages, 13 figures, Accepted for publication in Astrophsical
Journa
Chemical Enrichment in Damped Lyman Alpha Systems From Hierarchical Galaxy Formation Models
We investigate chemical enrichment in Damped Lyman alpha (DLA) systems in the
hierarchical structure formation scenario using a semi-analytic model of galaxy
formation. The model developed by Nagashima, Totani, Gouda and Yoshii takes
into account various selection effects on high-redshift galaxies and can show
fundamental observational properties of galaxies, such as luminosity functions
and number-magnitude/redshift relations. DLA systems offer the possibilities of
measuring metal abundance more accurately than faint galaxies. For example,
recent measurements of zinc abundance can provide good evidence for
understanding the processes of metal pollution and star formation in DLA
systems because zinc is virtually unaffected by dust depletion. Here we focus
on this advantage for observation in order to explore the metallicity evolution
in DLA systems at high redshifts. We can consistently show the metallicity
evolution for reasonable models which also reproduce fundamental properties of
local galaxy population. This result suggests that the chemical evolution of
DLA systems can be consistently reconciled with the observational features of
typical galaxies. We also investigate other properties of DLA systems (column
density distribution and mass density of cold gas), and find that star
formation in massive galaxies should be more active than that in low-mass ones.
This is consistent with the results by Nagashima et al. and Cole et al. in
which the star formation timescale is set by reproducing cold gas mass fraction
in local spiral galaxies. Finally we discuss host galaxies associated with DLA
systems. We conclude that they primarily consist of sub-L* and/or dwarf
galaxies from the observations.Comment: 16 pages, 11 figures, Accepted for publication in Astrophsical
Journa