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Register a new userThe chemical evolution of galaxies causing damped Ly$alpha$ absorption
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We have compiled all available data on chemical abundances in damped Lyman alpha absorption systems for comparison with results from our combined chemical and spectrophotometric galaxy evolution models. Preliminary results from chemically consistent calculations are in agreement with observations of damped Ly$alpha$ systems.
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We calculate in detail the expected properties of low redshift DLA systems under the assumption that they arise in the gaseous disks of galaxies like those in the z=0 population. A sample of 355 nearby galaxies is analysed, for which high quality HI 21-cm emission line maps are available as part of an extensive survey with the Westerbork telescope (WHISP). We find that expected luminosities, impact parameters between quasars and DLA host galaxies, and metal abundances are in good agreement with the observed properties of DLAs and DLA galaxies. The measured redshift number density of z=0 gas above the DLA limit is dN/dz=0.045 +/- 0.006, which compared to higher redshift measurements implies that there is no evolution in the comoving density of DLAs along a line of sight between z=1.5 and z=0, and a decrease of only a factor of two from z=4 to the present time. We conclude that the local galaxy population can explain all properties of low redshift DLAs.
We study the average Ly$alpha$ emission associated with high-$z$ strong (log $N$(H I) $ge$ 21) damped Ly$alpha$ systems (DLAs). We report Ly$alpha$ luminosities ($L_{rm Lyalpha}$) for the full as well as various sub-samples based on $N$(H I), $z$, $(r-i)$ colours of QSOs and rest equivalent width of Si II$lambda$1526 line (i.e., $W_{1526}$). For the full sample, we find $L_{rm Lyalpha}$$< 10^{41} (3sigma) rm erg s^{-1}$ with a $2.8sigma$ level detection of Ly$alpha$ emission in the red part of the DLA trough. The $L_{rm Lyalpha}$ is found to be higher for systems with higher $W_{1526}$ with its peak, detected at $geq 3sigma$, redshifted by about 300-400 $rm km s^{-1}$ with respect to the systemic absorption redshift, as seen in Lyman Break Galaxies (LBGs) and Ly$alpha$ emitters. A clear signature of a double-hump Ly$alpha$ profile is seen when we consider $W_{1526} ge 0.4$ AA and $(r-i) < 0.05$. Based on the known correlation between metallicity and $W_{1526}$, we interpret our results in terms of star formation rate (SFR) being higher in high metallicity (mass) galaxies with high velocity fields that facilitates easy Ly$alpha$ escape. The measured Ly$alpha$ surface brightness requires local ionizing radiation that is 4 to 10 times stronger than the metagalactic UV background at these redshifts. The relationship between the SFR and surface mass density of atomic gas seen in DLAs is similar to that of local dwarf and metal poor galaxies. We show that the low luminosity galaxies will contribute appreciably to the stacked spectrum if the size-luminosity relation seen for H I at low-$z$ is also present at high-$z$. Alternatively, large Ly$alpha$ halos seen around LBGs could also explain our measurements.
We used our database of ESO VLT-UVES spectra of quasars to build up a sample of 67 Damped Lyman-alpha (DLA) systems with redshifts 1.7<zabs<3.7. For each system, we measured average metallicities relative to Solar, [X/H] (with either X=Zn, S or Si), and the velocity widths of low-ionization line profiles, W1. We find that there is a tight correlation between the two quantities, detected at the 5sigma significance level. The existence of such a correlation, over more than two orders of magnitude spread in metallicity, is likely to be the consequence of an underlying mass-metallicity relation for the galaxies responsible for DLA absorption lines. The best-fit linear relation is [X/H]=1.35(pm 0.11)log W1 -3.69(pm 0.18)$ with W1 expressed in km/s. While the slope of this velocity-metallicity relation is the same within uncertainties between the higher and the lower redshift bins of our sample, there is a hint of an increase of the intercept point of the relation with decreasing redshift. This suggests that galaxy halos of a given mass tend to become more metal-rich with time. Moreover, the slope of this relation is consistent with that of the luminosity-metallicity relation for local galaxies. The DLA systems having the lowest metallicities among the DLA population would therefore, on average, correspond to the galaxies having the lowest masses. In turn, these galaxies should have the lowest luminosities among the DLA galaxy population. This may explain the recent result that the few DLA systems with detected Ly-alpha emission have higher than average metallicities.
We investigate the properties of damped Ly{alpha} absorption systems (DLAs) in semi-analytic models of galaxy formation, including partitioning of cold gas in galactic discs into atomic, molecular, and ionized phases with a molecular gas-based star formation recipe. We investigate two approaches for partitioning gas into these constituents: a pressure-based and a metallicity-based recipe. We identify DLAs by passing lines of sight through our simulations to compute HI column densities. We find that models with standard gas radial profiles - where the average specific angular momentum of the gas disc is equal to that of the host dark matter halo - fail to reproduce the observed column density distribution of DLAs. These models also fail to reproduce the distribution of velocity widths {Delta}v, overproducing low {Delta}v relative to high {Delta}v systems. Models with extended radial gas profiles - corresponding to gas discs with higher specific angular momentum - are able to reproduce quite well the column density distribution of absorbers over the column density range 19 < log NHI < 22.5 in the redshift range 2 < z < 3.5. The model with pressure-based gas partitioning also reproduces the observed line density of DLAs, HI gas density, and {Delta}v distribution at z < 3 remarkably well. However all of the models investigated here underproduce DLAs and the HI gas density at z > 3. If this is the case, the flatness in the number of DLAs and HI gas density over the redshift interval 0 < z < 5 may be due to a cosmic coincidence where the majority of DLAs at z > 3 arise from intergalactic gas in filaments while those at z < 3 arise predominantly in galactic discs. We further investigate the dependence of DLA metallicity on redshift and {Delta}v, and find reasonably good agreement with the observations, particularly when including the effects of metallicity gradients (abbrv.).
In 2000, we started the program ``Building the Bridge between Damped Ly-alpha Absorbers and Lyman-Break Galaxies: Ly-alpha Selection of Galaxies at the European Southern Observatorys Very Large Telescope. This project is an attempt to use Ly-alpha selection of high-z galaxies to bridge the gap between absorption- and emission-selected galaxies by creating a large database of z=3 galaxies belonging to the abundant population of faint (R>25.5) galaxies probed by the Damped Ly-alpha Absorbers (DLAs). Here we present the first results of our program, namely the results from a deep Ly-alpha study of the field of the z=2.85 DLA towards Q2138-4427.
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