Starcounts Redivivus. III. A Possible Detection of the Sagittarius Dwarf Spheroidal Galaxy at b = −40°

As part of the Selected Areas Starcounts Survey, a CCD survey to V > 21, we have obtained VI photometry of two fields at b = ±40° aligned roughly with an extrapolation of the major axis of the Sagittarius dwarf spheroidal galaxy. Comparison of the color-magnitude diagram (CMD) for some of these fields with the CMDs of fields reflected about the Galactic l = 0° meridian reveals an excess of stars at V0 = 17.85 and 0.9 < (V-I)0 < 1.1 in the (l, b) = (11°, -40°) field. The excess stars have colors consistent with the Sgr red clump, and deeper CMD imaging in these locations shows evidence of a main-sequence turnoff (MSTO) at V = 21, with the main sequence extending to the limit of our data (V = 24). The surface brightnesses we derive from either the potential excess of red clump stars or the apparent excess of MSTO stars are consistent with each other and with the results of other surveys at this latitude. No similar excess appears in our northern Galactic hemisphere fields near the (l, b) = (353°, +41°) field. We have obtained spectroscopy of all 30 candidate red clump stars in the range 0.9 < (V-I)0 < 1.1and 17.75 < V0 < 17.95. The radial velocity distribution of the stars, while dissimilar from expectations of Galactic structure models, does not show a contribution by stars near the Galactocentric radial velocity seen in other studies near the Sgr core. It is difficult to reconcile a photometric result that is consistent with other explorations of the Sagittarius stream with a radial velocity distribution that is apparently inconsistent. In a companion paper, we discuss how some of the discrepancies are resolved if our potential Sgr detection corresponds to a different Sgr tidal streamer than that detected by most other surveys.