Tracing the Outer Structure of the Sagittarius Dwarf Galaxy:Detections at Angular Distances between 10° and 34°

Abstract

We have obtained deep photometric data in 24 fields along the southeast extension of the major axis of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy and in four fields along the northwest extension. Using star counts at the expected position of the upper main sequence within the resulting color-magnitude diagrams, we unambiguously detect stars in the southeast over the range 10°-34° from the galaxy's center. If is symmetric, this implies a true major-axis diameter of at least 68°, or nearly 30 kpc if all portions of are equally distant from the Sun. Star counts parallel to the galaxy's minor axis reveal that remains quite broad far from its center. This suggests that the outer portions of resemble a stream rather than an extension of the ellipsoidal inner regions of the galaxy. The inferred V-band surface brightness (SB) profile ranges from 27.3 to 30.5 mag arcsec-2 over this radial range and exhibits a change in slope ~20° from the center of Sgr. The scale length of the outer SB profile is 172, compared with 47 in the central region of Sgr. We speculate that this break in the SB profile represents a transition from the main body of to a more extended Sgr stream. By integrating the SB profile, we estimate that the absolute visual magnitude of lies in the range -13.4 to -14.6, depending on the assumed structure of Sgr; an upper limit to the luminosity of is therefore L~5.8×107 L☉. This result lowers the M/LV ratio inferred for down to ~10, which is consistent with values observed in the most luminous dSph companions of the Milky Way.