Journal Article

Chemical abundances in the young galaxy at <i>z</i>=2.309 towards PHL 957

P. Molaro, M. Centurión and G. Vladilo

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 293, issue 1, pages L37-L41
Published in print January 1998 | ISSN: 0035-8711
Published online January 1998 | e-ISSN: 1365-2966 | DOI:
Chemical abundances in the young galaxy at z=2.309 towards PHL 957

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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 Lyα (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 ≈74 per cent of iron and ≈28 per cent of zinc are locked into dust grains with a dust-to-gas ratio of ≈3 per cent 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. 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 [α/Fe] values do not support Galactic halo-like abundances, implying that the chemical evolution of this young galaxy is not reproducing the evolution of our own Galaxy.

Keywords: stars: abundances; stars: Population II; Galaxy: halo; cosmology: observations

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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