According to Pei , Fall & Hauser ( 1999 ) , the global metallicity evolution of the Universe can be represented by the ratio of the total metal content to the total gas content measured in Damped Lyman– \alpha ( DLA ) systems ( the “ column density weighted metallicity ” à la Pettini ) .
To minimize dust obscuration effects , a DLA sample with negligible dust content is considered , namely , 50 DLAs with \log N _ { HI } < 20.8 .
The global metallicity found shows clear evidence of redshift evolution that goes from \sim 1 / 30 solar at z \sim 4.1 to solar at z \sim 0.4 .
More generally , DLAs with measured heavy elements probe the ISM of high redshift galaxies .
The whole sample collected from the literature contains 75 DLAs .
The metallicity is calculated adopting for the dust correction the most general method used so far , based on models of the ISM dust depletions in the Galaxy .
The intrinsic metallicity evolution of DLA galaxies is d \log Z _ { DLA } / dz \propto - 0.33 \pm 0.06 .