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

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 Sgr upper main-sequence within the resulting color-magnitude diagrams (CMDs), we unambiguously detect Sgr stars in the southeast over the range 10--34 degreesfrom the galaxy's center. If Sgr is symmetric, this implies a true major-axis diameter of at least 68 degrees, or nearly 30 kpc if all portions of Sgr are equally distant from the Sun. Star counts parallel to the galaxy's minor-axis reveal that Sgr remains quite broad far from its center. This suggests that the outer portions of Sgr 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 27.3-30.5 mag per arcsec**2 over this radial range and exhibits a change in slope approx 20 degrees from the center of Sgr. The scale length of the outer SB profile is 17.2 degrees, compared to 4.7 degrees in the central region of Sgr. We speculate that this break in the SB profile represents a transition from the main body of Sgr to a more extended `Sgr stream'. By integrating the SB profile we estimate that the absolute visual magnitude of Sgr lies in the range -13.4 to -14.6, depending on the assumed structure of Sgr; an upper limit to the luminosity of Sgr is therefore approx 5.8x10**7 Solar Luminosities. This result lowers the V-band M/L ratio inferred for Sgr by Ibata et al. (1997) down to approx 10, consistent with values observed in the most luminous dSph companions of the Milky Way.