The Sgr dSph hosts a metal-rich population

Free access article

Issue		A&A Volume 414, Number 2, February I 2004


Page(s)		503 - 514
Section		Galactic structure and dynamics
DOI		http://dx.doi.org/10.1051/0004-6361:20031692

A&A 414, 503-514 (2004)
DOI: 10.1051/0004-6361:20031692

The Sgr dSph hosts a metal-rich population

P. Bonifacio¹, L. Sbordone^{2, 3, 4}, G. Marconi⁴, L. Pasquini⁴ and V. Hill⁵

¹ Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
² Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Roma, Italy Via Frascati 33, 00040 Monteporzio Catone, Roma, Italy
³ Università Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴ European Southern Observatory, Casilla 19001, Santiago, Chile
⁵ Observatoire de Paris-Meudon, 2 pl. J. Janssen, 92190 Meudon, France

(Received 2 May 2003 / Accepted 28 October 2003)

Abstract
We report on abundances of O, Mg, Si, Ca and Fe for 10 giants in the Sgr dwarf spheroidal derived from high resolution spectra obtained with UVES at the 8.2 m Kueyen-VLT telescope. The iron abundance spans the range $\rm -0.8 \la [Fe/H] \la 0.0$ and the dominant population is relatively metal-rich with $\rm [Fe/H]\sim -0.25$ . The $\alpha$ /Fe ratios are slightly subsolar, even at the lowest observed metallicities suggesting a slow or bursting star formation rate. From our sample of 12 giants (including the two observed by Bonifacio et al. 2000) we conclude that a substantial metal rich population exists in Sgr, which dominates the sample. The spectroscopic metallicities allow one to break the age-metallicity degeneracy in the interpretation of the colour-magnitude diagram (CMD). Comparison of isochrones of appropriate metallicity with the observed CMD suggests an age of 1 Gyr or younger, for the dominant Sgr population sampled by us. We argue that the observations support a star formation that is triggered by the passage of Sgr through the Galactic disc, both in Sgr and in the disc. This scenario has also the virtue of explaining the mysterious "bulge C stars" as disc stars formed in this event. The interaction of Sgr with the Milky Way is likely to have played a major role in its evolution.

Key words: stars: abundances -- stars: atmospheres -- galaxies: abundances -- galaxies: evolution -- galaxies: dwarf -- galaxies: individual: Sgr dSph

Offprint request: P. Bonifacio, bonifaci@ts.astro.it

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