Chemical abundances in the young galaxy at z=2.309 towards PHL 957

We present high-resolution UES spectra of the quasar PHL 957 obtained for studying the foreground Damped Ly-alpha galaxy at z=2.309. Measurements of absorption lines lead to accurate abundance determinations of Fe, S and N which complement measurements of Zn, Cr and Ni already available for this system. We find [Fe/H]= -2.0 (+/-0.1), [S/H]= -1.54 (+/-0.06) and [N/H]=-2.76 (+/-0.07). The ratio [Fe/Zn]=-0.44 provides evidence that about 74% of iron and about 28% of zinc are locked into dust grains with a dust-to-gas ratio of about 3% of the Galactic one. The total iron content in both gas and dust in the DLA system is [Fe/H]=-1.4. This confirms a rather low metallicity in the galaxy, which is in the early stages of its chemical evolution. The detection of SII allows us to measure the SII/ZnII ratio, which is a unique diagnostic tool for tracing back its chemical history, since it is not affected by the presence of dust. Surprisingly, the resulting relative abundance is [S/Zn]=0.0 (+/-0.1), at variance with the overabundance found in the Galactic halo stars with similar metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA absorbers with extant data. Upper limits are also found for Mn, Mg, O and P and, once the dust depletion is accounted for, we obtain [Mg/Fe]_{c} <+0.2, [O/Fe]_{c}< 0.4, [Mn/Fe]_{c}<+0.0 and [P/Fe]_{c}<-0.7. The [alpha/Fe] values do not support a Galactic halo-like abundances implying that the chemical evolution of this young galaxy is not reproducing our own Galaxy's evolution.