We present Iron , Magnesium , Calcium , and Titanium abundances for 235 stars in the central region of the Sagittarius dwarf spheroidal galaxy ( within 9.0 \arcmin \simeq 70 pc from the center ) from medium-resolution Keck/DEIMOS 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 ] \geq - 1.0 , more than doubling the available sample of spectroscopic metallicity and \alpha -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 \leq 2.5 \arcmin to [ Fe/H ] = -0.57 for 5.0 < R \leq 9.0 \arcmin .
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 implying a high mass progenitor ( M _ { DM } = 6 \times 10 ^ { 10 } ~ { } M _ { \sun } ) and a significant event of mass-stripping occurred a few Gyr ago , presumably starting at the first peri-Galactic passage after infall .