The early build-up of dust in galaxies:: A study of damped Ly α 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 Lyalpha 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 less than or equal to z less than or equal to 3.4 selected on the basis of their absorption H I column density (N(HI) greater than or equal to2 x 10(20) atoms cm(-2)). We find that the dust fraction increases with metallicity, from f(Fe) similar or equal to 0 at [Fe/H] similar to -2 dex, up to f(Fe) similar or equal to 0.9 at solar metallicity; the increase is fast below [Fe/H] similar or equal to -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 approximate to3 Gyr has f(Fe) less than or similar to 0.5. These results indicate that 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] greater than or similar to -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 to explain the rise of f(Fe) at [Fe/H] less than or similar to -1 dex.