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| - The early build-up of dust in galaxies: A study of damped Ly α systems
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| - 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 α systems (DLAs) for which we derive the fraction of iron atoms in dust form, fFe. The sample is representative of metal-poor galaxies in the redshift range $ 0.6 \leq z \leq 3.4$ selected on the basis of their absorption $\ion{H}{i}$ column density
( $N(\ion{H}{i}) \geq 2 \times 10^{20}$ atoms cm -2).
We find that the dust fraction increases with metallicity, from $f_{\rm Fe} \simeq 0$ at $\mbox{[Fe/H]}\sim -2$ dex, up to $f_{\rm Fe} \simeq 0.9$ at solar metallicity; the increase is fast below $\mbox{[Fe/H]} \simeq -1$ dex and mild at higher metallicities. We also find some evidence for an increase of fFe with cosmic time; a large fraction of the systems younger than ≈3 Gyr has $f_{\rm Fe} \la 0.5$. 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 ( $\mbox{[Fe/H]}\ga -1$ 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 fFe at $\mbox{[Fe/H]} \la -1$ dex.
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