315 research outputs found
Estimating the mass density of neutral gas at
We use the relationships between galactic HI mass and B-band luminosity
determined by Rao & Briggs to recalculate the mass density of neutral gas at
the present epoch based on more recent measures of the galaxy luminosity
function than were available to those authors. We find in good agreement with the original Rao & Briggs
value, suggesting that this quantity is now reasonably secure. We then show
that, if the scaling between H I mass and B-band luminosity has remained
approximately constant since , the evolution of the luminosity function
found by the Canada-France redshift survey translates to an increase of
by a factor of at . A similar value is
obtained quite independently from consideration of the luminosity function of
Mg II absorbers at . By combining these new estimates with data from
damped \lya systems at higher redshift, it is possible to assemble a rough
sketch of the evolution of over the last 90% of the age of the
universe. The consumption of H I gas with time is in broad agreement with
models of chemical evolution which include the effects of dust, although more
extensive samples of damped \lya systems at low and intermediate redshift are
required for a quantitative assessment of the dust bias.Comment: LaTeX file, 11 pages, 1 figure, accepted MNRAS pink page
Near-pristine gas at high redshifts: a window on early nucleosynthesis
It has now become recognised that damped Lyman alpha systems play an
important role in helping us unravel the origin of chemical elements. In this
presentation, we describe the main results of a recently completed survey of
the most metal-poor DLAs, aimed at complementing and extending studies of the
oldest stars in the Galaxy. The survey has clarified a number of lingering
issues concerning the abundances of C, N, O in the low metallicity regime, has
revealed the existence of DLA analogues to Carbon-enhanced metal-poor stars,
and is providing some of the most precise measures of the primordial abundance
of Deuterium.Comment: 11 pages, 7 Figures. Invited presentation at the XII International
Symposium on Nuclei in the Cosmos, Cairns, Australia, 5-10 August 2012. To
appear in Proceedings of Scienc
The star formation rate of CaII and damped Lyman-alpha absorbers at 0.4<z<1.3
[abridged] Using stacked Sloan Digital Sky Survey spectra, we present the
detection of [OII]3727,3730 nebular emission from galaxies hosting CaII and
MgII absorption line systems. Both samples of absorbers, 345 CaII systems and
3461 MgII systems, span the redshift interval 0.4 < z < 1.3; all of the former
and half the latter sample are expected to be bona-fide damped Lyman-alpha
(DLA) absorbers. The measured star formation rate (SFR) per absorber from light
falling within the SDSS fibre apertures (corresponding to physical radii of 6-9
h^-1 kpc) is 0.11-0.14 Msol/yr for the MgII-selected DLAs and 0.11-0.48 Msol/yr
for the CaII absorbers. These results represent the first estimates of the
average SFR in an absorption-selected galaxy population from the direct
detection of nebular emission. Adopting the currently favoured model in which
DLAs are large, with radii >9h^-1 kpc, and assuming no attenuation by dust,
leads to the conclusion that the SFR per unit area of MgII-selected DLAs falls
an order of magnitude below the predictions of the Schmidt law, which relates
the SFR to the HI column density at z~0. The contribution of both DLA and CaII
absorbers to the total observed star formation rate density in the redshift
range 0.4 < z < 1.3, is small, <10% and <3% respectively. The result contrasts
with the conclusions of Hopkins et al. that DLA absorbers can account for the
majority of the total observed SFR density in the same redshift range. Our
results effectively rule out a picture in which DLA absorbers are the sites in
which a large fraction of the total SFR density at redshifts z < 1 occurs.Comment: Accepted for publication in MNRAS, 13 pages, 6 figure
Mixing Metals in the Early Universe
We investigate the evolution of the metallicity of the intergalactic medium
(IGM) with particular emphasis on its spatial distribution. We propose that
metal enrichment occurs as a two step process. First, supernova (SN) explosions
eject metals into relatively small regions confined to the surroundings of
star-forming galaxies. From a comprehensive treatment of blowout we show that
SNae by themselves fail by more than one order of magnitude to distribute the
products of stellar nucleosynthesis over volumes large enough to pollute the
whole IGM to the metallicity levels observed. Thus, a additional (but as yet
unknown) physical mechanism must be invoked to mix the metals on scales
comparable to the mean distance between the galaxies which are most efficient
pollutants. From this simple hypothesis we derive a number of testable
predictions for the evolution of the IGM metallicity. Specifically, we find
that: (i) the fraction of metals ejected over the star formation history of the
universe is about 50% at z=0; that is, approximately half of the metals today
are found in the IGM; (ii) if the ejected metals were homogeneously mixed with
the baryons in the universe, the average IGM metallicity would be ~ 1/25
Z(solar) at z=3. However, due to spatial inhomogeneities, the mean of the
distribution of metallicities in the diffusive zones has a wide (more than 2
orders of magnitude) spread around this value; (iii) if metals become more
uniformly distributed at z < 1, as assumed, at z = 0 the metallicity of the IGM
is narrowly confined within the range Z ~ 0.1 +/- 0.03 Z(solar). Finally, we
point out that our results can account for the observed metal content of the
intracluster medium.Comment: 26 pages, LaTeX, 8 figures, accepted by MNRA
- …