The Optical Gravitational Lensing Experiment. Deep Photometry of the Sagittarius Dwarf Spheroidal Galaxy

We present deep CCD photometry of a field in the newly discovered dwarf spheroidal galaxy in Sagittarius (hereafter Sgr), and of a nearby control field. These data (totaling ~3.5 hrs exposure time in V and I) were used to produce color-magnitude (CM) diagrams reaching I ~ 22.3 in both fields. After statistically removing the field-stars from the CM diagram we find that Sgr is dominated by a moderately old (age ~ 10 Gyr) population, significantly younger than a typical globular-cluster population. There is some evidence for a weak intermediate-age component or alternatively a population of blue stragglers. These results confirm that Sgr is a bona fide dwarf spheroidal (dSph) galaxy, the ninth found orbiting the Milky Way. We have discovered nine short-period variables in the Sgr field, well in excess of the number found in the control field. Seven of these stars have similar apparent magnitudes and are almost certainly RR~Lyr stars in Sgr. We have used the minimum-light colors of the RRab variables to determine the reddening of the Sgr field to be E\vi\ = 0.22. For an assumed RR~Lyr luminosity of M_V = +0.6 we conclude that the distance of Sgr is 25.2 +/- 2.8 kpc. The properties of the Sgr giant branch and upper main sequence are consistent with a mean metallicity of [Fe/H] ~ -1.1 +/- 0.3.