A wide angle view of the Sagittarius dwarf Spheroidal Galaxy. I: VIMOS
  photometry and radial velocities across Sgr dSph major and minor axis
  The Sagittarius dwarf Spheroidal Galaxy (Sgr dSph) provides us with a unique
possibility of studying a dwarf galaxy merging event while still in progress.
Due to its low distance (25 kpc), the main body of Sgr dSph covers a vast area
in the sky (roughly 15 x 7 degrees). Available photometric and spectroscopic
studies have concentrated either on the central part of the galaxy or on the
stellar stream, but the overwhelming majority of the galaxy body has never been
probed. The aim of the present study is twofold. On the one hand, to produce
color magnitude diagrams across the extension of Sgr dSph to study its stellar
populations, searching for age and/or composition gradients (or lack thereof).
On the other hand, to derive spectroscopic low-resolution radial velocities for
a subsample of stars to determine membership to Sgr dSph for the purpose of
high resolution spectroscopic follow-up. We used VIMOS-VLT to produce V and I
photometry and spectroscopy on 7 fields across the Sgr dSph minor and major
axis, plus 3 more centered on the associated globular clusters Terzan 7, Terzan
8 and Arp 2. A last field has been centered on M 54, lying in the center of Sgr
dSph. We present photometry for 320,000 stars across the main body of Sgr dSph,
one of the richest, and safely the most wide-angle sampling ever produced for
this fundamental object. We also provide robust memberships for more than one
hundred stars, whose high resolution spectroscopic analysis will be the object
of forthcoming papers. Sgr dSph appears remarkably uniform among the observed
fields. We confirm the presence of a main Sgr dSph population characterized
roughly by the same metallicity of 47 Tuc, but we also found the presence of
multiple populations on the peripheral fields of the galaxy, with a metallicity
spanning from [Fe/H]=-2.3 to a nearly solar value.
