The Sagittarius Dwarf Galaxy Survey (SDGS) -- I. Colour--magnitude diagrams, reddening and population gradients. First evidence of a very metal-poor population

Abstract

We present the first results of a large photometric survey devoted to the study of the star formation history of the Sagittarius dwarf spheroidal galaxy (Sgr dSph). Three wide strips (size ∼ 9 × 35 arcmin 2) located at ∼ (l ° b °) = (6.5;-16), (6;-14), (5;-12) have been observed. Each strip is roughly east--west oriented, nearly along the major axis of the galaxy. A control field (size ∼ 9 × 24 arcmin 2), located outside the body of Sgr dSph [∼ (l ° b °) = (354;-14)] has also been observed for statistical decontamination purposes. Accurate and well-calibrated V, I photometry down to V ∼ 22 has been obtained for ∼ 90 000 stars towards 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 at a metallicity gradient towards the densest region of the galaxy are also reported.