A new comprehensive set of elemental abundances in DLAs^*

III. Star formation histories

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: miroslava.dessauges@obs.unige.ch
² INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
³ UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
⁴ European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching bei München, Germany
⁵ Dipartimento di Astronomia, Universitá di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy

Received: 3 January 2007
Accepted: 9 May 2007

Abstract

We obtained comprehensive sets of elemental abundances for eleven damped Lyα systems (DLAs) at z_DLA = 1.7-2.5. For nine of them, we accurately constrained their intrinsic abundance patterns accounting for dust depletion and ionization effects. In Paper I of this series (Dessauges-Zavadsky et al. 2004, A&A, 416, 79), we showed for three DLA galaxies that we can derive their star formation histories and ages from a detailed comparison of their intrinsic abundance patterns with chemical evolution models. We determine in this paper the star formation properties of six additional DLA galaxies. The derived results confirm that no single star formation history explains the diverse sets of abundance patterns in DLAs. We demonstrate that the various star formation histories reproducing the DLA abundance patterns are typical of local irregular, dwarf starburst and quiescent spiral galaxies. Independent of the star formation history, the DLAs have a common characteristic of being weak star forming galaxies; models with high star formation efficiencies are ruled out. The distribution of the DLA star formation histories shows a trend of finding more galaxies with a star formation history typical of dwarf irregulars with a bursting star formation toward high redshifts, . Only two DLA galaxies (each at ) in our sample of nine objects have a star formation history typical of spiral galaxies. Since DLAs sample the broad distribution of galaxies at high redshift, this trend indicates that young and less evolved proto-galactic structures with low masses and low star formation rates are more common toward higher redshifts. This is further supported by the star formation rate and age distributions. Indeed, all the derived DLA star formation rates per unit area are moderate or low, with values between -3.2 < log SFR < -1.1  yr^-1 kpc^-2. The DLA abundance patterns also require a large spread in ages ranging from 20 Myr up to 3 Gyr. Enhanced α over iron-peak ratios are associated with young objects having undergone a recent burst of star formation, while solar α over iron-peak ratios are associated with old objects undergoing an inefficient continuous star formation. The oldest DLA in our sample is observed at z_DLA = 1.864 with an age estimated to more than 3 Gyr; it nicely indicates that galaxies were already forming at 10. But, most of the DLAs show ages much younger than that of the Universe at the epoch of observation. Young galaxies thus seem to populate the high redshift Universe at , suggesting relatively low redshifts of formation (z ~ 3) for most high-redshift galaxies. The large dispersion in star formation history and age indicates that the DLAs are drawn from a diverse population of galaxies. The DLA star formation properties are compared with those of other high-redshift galaxies identified in deep imaging surveys with the aim of obtaining a global picture of high-redshift objects.