The early build-up of dust in galaxies: A study of Damped Ly alpha Systems

We present a study of the early build-up of dust in high redshift galaxies. The study is based on the analysis of 38 Damped Ly alpha systems (DLAs) for which we derive the fraction of iron atoms in dust form, f_{Fe}. The sample is representative of metal-poor galaxies in the redshift range 0.6 </= z </= 3.4 selected on the basis of their absorption HI column density (N(HI) >/= 2 x 10^{20} atoms cm^{-2}). We find that the dust fraction increases with metallicity, from f_{Fe}~0 at [Fe/H] ~ -2 dex, up to f_{Fe} ~ 0.9 at solar metallicity; the increase is fast below [Fe/H] ~ -1 dex and mild at higher metallicities. We also find some evidence for an increase of f_{Fe} with cosmic time; a large fraction of the systems younger than ~3 Gyr has f_{Fe} </~ 0.5. These results indicate the dust-to-metal ratio increases in the course of chemical evolution, at variance with the hypothesis of an approximately constant dust-to-metal ratio, commonly adopted in models of galactic evolution. This hypothesis is consistent with local and high-redshift data only when the metallicity is relatively high ([Fe/H] >/~ -1 dex). The results of this work suggest that the main mechanisms of dust formation may be rather sensitive to the level of metallicity attained by a galaxy in the course of its chemical evolution. A metallicity-dependent dust production by SNe II seems to be the most promising mechanism for explaining the rise of f_{Fe} at [Fe/H] </~ -1 dex.Comment: Accepted for publication on Astronomy & Astrophysics; 13 pages, 5 figure

A scaling law for interstellar depletions

An analytical expression is presented that allows gas-to-dust elemental depletions to be estimated in interstellar environments of different types, including Damped Ly alpha systems, by scaling an arbitrary depletion pattern chosen as a reference. As an improvement on previous work, the scaling relation allows the dust chemical composition to vary and includes a set of parameters which describe how sensitive the dust composition is to changes in both the dust-to-metals ratio and the composition of the medium. These parameters can be estimated empirically from studies of Galactic and extragalactic depletion patterns. The scaling law is able to fit all the typical depletion patterns of the Milky Way ISM (cold disk, warm disk, and warm halo) with a single set of parameters, by only varying the dust-to-metals ratio. The dependence of the scaling law on the abundances of the medium has been tested using interstellar observations of the Small Magellanic Cloud (SMC), for which peculiar depletion patterns have been reported in literature. The scaling law is able to fit these depletion patterns assuming that the SMC relative abundances are slightly non solar.Comment: Accepted for publication in the Astrophysical Journal, 15 pages, 10 figure