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.