2000ApJ...536..540C

Relative elemental abundances, and in particular the α/Fe ratio, are important diagnostic tools of the chemical evolution of damped Lyα (DLA) systems. The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the α/Fe ratio. We report six new determinations of sulphur abundance in DLA systems at z_abs≥2 with already-known zinc abundances. The combination with extant data from the literature provides a measure of the S/Zn abundance ratio for a total of 11 high-redshift DLA systems. The observed [S/Zn] ratios do not show the characteristic [α/Fe] enhancement observed in metal-poor stars of the Milky Way at a comparable level of metallicity ([Zn/H]~-1). The behavior of DLA data is consistent with a general trend of a decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the α/Fe ratio in the course of the chemical evolution of DLA systems. However, in contrast to what is observed in our Galaxy, the α/iron peak ratio seems to attain solar values when the metallicity is still low ([Zn/H]≤-1) and to decrease below solar values at higher metallicities. The behavior of the α/Fe ratio challenges the frequently adopted hypothesis that high-redshift DLA systems are progenitors of spiral galaxies and instead favors an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low-redshift DLA systems, where the candidate DLA galaxies show a variety of morphological types, including dwarf and low surface brightness galaxies and only a minority of spirals.