The early build-up of dust in galaxies: A study of damped Ly α systems

Osservatorio Astronomico di Trieste - Istituto Nazionale di Astrofisica, via G.B. Tiepolo 11, 34131 Trieste, Italy

Corresponding author: vladilo@ts.astro.it

Received: 18 December 2003
Accepted: 10 March 2004

Abstract

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 Lyα 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 selected on the basis of their absorption column density
( atoms cm^-2). 
 We find that the dust fraction increases with metallicity, from at  dex, up to at solar metallicity; the increase is fast below  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 . 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 ( 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  dex.