Metallicity in damped Lyman-alpha systems: evolution or bias?

Assuming that damped Lyman-alpha(DLA) systems are galactic discs, we calculate the corresponding evolution of metal abundances. We use detailed multi-zone models of galactic chemical evolution (reproducing successfully the observed properties of disc galaxies) and appropriate statistics (including geometrical propability factors) to calculate the average metallicity as a function of redshift. The results are compatible with available observations, provided that observational biases are taken into account, as suggested by Boisse et al. (1998). In particular, high column density and high metallicity systems are not detected because the light of backround quasars is severely extinguished, while low column density and low metallicity systems are not detectable through their absorption lines by current surveys. We show that these observational constraints lead to a ``no-evolution'' picture for the DLA metallicity, which does not allow to draw strong conclusions about the nature of those systems or about their role in ``cosmic chemical evolution''.Comment: 7 pages, 5 figures, MNRAS in pres

Perspectives in Galactic Chemical Evolution studies

In this review I focus on a few selected topics, where recent theoretical and/or observational progress has been made and important developments are expected in the future. They include: 1) Evolution of isotopic ratios, 2) Mixing processes and dispersion in abundance ratios, 3) Abundance gradients in the Galactic disk (and abundance patterns in the inner Galaxy), 4) The question of primary Nitrogen and 5) Abundance patterns in extragalactic damped Lyman-alpha systems (DLAs)

Chemical evolution and depletion pattern in Damped Lyman-alpha (DLA) systems

In this paper we point out a previously unnoticed anticorrelation between the observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X stands for the refractories Fe, Cr and Ni) and metal column density ([Zn/H]+log(N{HI})) in DLAs. We suggest that this trend is an unambiguous sign of dust depletion, since metal column density is a measure of the amount of dust along the line of sight. Assuming that DLAs are (proto-)galactic disks and using detailed chemical evolution models with metallicity dependent yields we study chemical evolution and dust depletion patterns for alpha and iron-peak elements in DLAs. When observational constraints on the metal column density of DLAs are taken into account (as suggested in Boisse et al. 1998) we find that our models reproduce fairly well the observed mild redshift evolution of the abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the observed scatter at a given redshift. By considering the aforementioned dependence of abundance ratios on metal column density, we further explore the general dust depletion pattern in DLAs, comparing to our model results and to a solar reference pattern. We suggest that further measurements of the key elements, i.e. Zn, S and Mn, will help to gain more insight into the nature of DLAs. In any case, the presently uncertain nucleosynthesis of Zn in massive stars (on which a large part of these conclusions is based) should be carefully scrutinised.Comment: 12 pages, 4 figures, Astronomy and Astrophysics, in pres

Chemo-spectrophotometric evolution of spiral galaxies: III. Abundance and colour gradients in discs

We study the relations between luminosity and chemical abundance profiles of spiral galaxies, using detailed models for the chemical and spectro-photometric evolution of galactic discs. The models are ``calibrated'' on the Milky Way disc and are successfully extended to other discs with the help of simple ``scaling'' relations, obtained in the framework of semi-analytic models of galaxy formation. We find that our models exhibit oxygen abundance gradients that increase in absolute value with decreasing disc luminosity (when expressed in dex/kpc) and are independent of disc luminosity (when expressed in dex/scalelength), both in agreement with observations. We notice an important strong correlation between abundance gradient and disc scalelength. These results support the idea of ``homologuous evolution'' of galactic discs.Comment: 9 pages, 6 postscript figures, MNRAS in pres

{Chemo-spectrophotometric evolution of spiral galaxies: IV. Star formation efficiency and effective ages of spirals

We study the star formation history of normal spirals by using a large and homogeneous data sample of local galaxies. For our analysis we utilise detailed models of chemical and spectrophotometric galactic evolution, calibrated on the Milky Way disc. We find that star formation efficiency is independent of galactic mass, while massive discs have, on average, lower gas fractions and are redder than their low mass counterparts; put together, these findings convincingly suggest that massive spirals are older than low mass ones. We evaluate the effective ages of the galaxies of our sample and we find that massive spirals must be several Gyr older than low mass ones. We also show that these galaxies (having rotational velocities in the 80-400 km/s range) cannot have suffered extensive mass losses, i.e. they cannot have lost during their lifetime an amount of mass much larger than their current content of gas+stars.Comment: 11 pages, 8 figures, MNRAS in pres