The Sagittarius Dwarf Galaxy Survey (SDGS) I: CMDs, Reddening and Population Gradients. First evidences of a very metal poor population

Abstract

We present first results of a large photometric survey devoted to study the star formation history of the Sagittarius dwarf spheroidal galaxy (Sgr dSph). Three wide strips (9 X 35 arcmin^2) located at l~5 and b=-16, -14, -12 have been observed. Each strip is roughly EW oriented, nearly along the major axis of the galaxy. A control field (~9 X 24 arcmin^2), located outside the body of Sgr dSph has also been observed for statistic decontamination purposes. Accurate and well calibrated V,I photometry down to V~22 have been obtained for \~90000 stars toward the Sgr dSph, and ~8000 stars in the control field. This is the largest photometric sample (covering the widest spatial extension) ever observed in the Sgr dSph up to now. The main new results presented in this paper are: (1) the possible discovery of a strong asymmetry in the distribution of stars along the major axis, since the north-western arm of the Sgr galaxy (i.e. the region nearer to the Galactic Bulge) apparently shows a significant deficiency of Sgr stars and (2) the first direct detection of a very metal poor (and presumably old) population in the Sgr stellar content. Hints for a metallicity gradient toward the densest region of the galaxy are also reported.