Chemical evolution of damped Ly alpha galaxies: The [S/Zn] abundance ratioat redshift >= 2

Relative elemental abundances, and in particular the alpha/Fe ratio, are im portant diagnostic tools of the chemical evolution of damped Ly alpha (DLA) systems. The S/Zn ratio is not affected by differential, dust depletion an d is an excellent estimator of the alpha/Fe ratio. We report six new determ inations of sulphur abundance in DLA systems at z(abs) greater than or equa l to 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 t otal of 11 high-redshift DLA systems. The observed [S/Zn] ratios do not sho w the characteristic [alpha/Fe] enhancement observed in metal-poor stars of the Milky Way at a comparable level of metallicity ([Zn/H] approximate to - 1). The behavior of DLA data is consistent with a general trend of a decr easing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the f irst evidence of the expected decrease of the alpha/Fe ratio in the course of the chemical evolution of DLA systems. However, in contrast to what is o bserved in our Galaxy, the alpha/iron peak ratio seems to attain solar valu es when the metallicity is still low ([Zn/H] less than or equal to - 1) and to decrease below solar values at higher metallicities. The behavior of th e alpha/Fe ratio challenges the frequently adopted hypothesis that high-red shift 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 th e candidate DLA galaxies show a variety of morphological types, including d warf and low surface brightness galaxies and only a minority of spirals.