Chemical abundances in the nucleus of the Sagittarius dwarf spheroidal galaxy∗

We present iron, magnesium, calcium, and titanium abundances for 235 stars in the central region of the Sagittarius dwarf spheroidal galaxy (within 9:00∼70 pc from the centre) from medium-resolution Keck/Deep Imaging Multi-Object Spectrograph spectra. All the considered stars belong to the massive globular cluster M 54 or to the central nucleus of the galaxy (Sgr, N). In particular we provide abundances for 109 stars with [Fe/H] ≥-1:0, more than doubling the available sample of spectroscopic metallicity and-elements abundance estimates for Sgr dSph stars in this metallicity regime.We find for the first time a metallicity gradient in the Sgr,N population, whose peak iron abundance goes from [Fe/H] =-0:38 for R ≤2:50 to [Fe/H] =-0:57 for 5:0 < R 9:0. On the other hand, the trends of [Mg/Fe], [Ca/Fe], and [Ti/Fe] with [Fe/H] are the same over the entire region explored by our study.We reproduce the observed chemical patterns of the Sagittarius dwarf spheroidal as a whole with a chemical evolution model which implies that a high mass progenitor (MDM = 6 1010 M) and a significant event of mass-stripping occurred a few Gyr ago, presumably starting at the first peri-Galactic passage after infall.