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

Molaro, P.; Vladilo, G.; Centurion, M.
Bibliographical reference

Monthly Notices of the Royal Astronomical Society, vol. 293, p. L37-L41

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We present high-resolution Utrecht Echelle Spectrograph spectra of the quasar PHL 957, obtained in order to study the foreground damped Ly-alpha (DLA) 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 percent of iron and about 28 percent of zinc are locked into dust grains with a dust-to-gas ratio of about 3 percent 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 S II allows us to measure the S II/Zn II ratio, which is a unique diagnostic tool for tracing back its chemical history, since it is not affected by the presence of dust.