A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms

We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxymapped by M giant star tracers detected in the complete Two Micron All-SkySurvey (2MASS). The main body is fit with a King profile of 30 deg limitingradius, but with a break in the density profile from stars in tidal tails. Weargue that much of the observed structure beyond the 224' core radius may beunbound as the satellite undergoes catastrophic disruption. A striking, >150deg trailing tidal tail extends from the Sgr center and arcs across the SouthGalactic Hemisphere. A prominent leading debris arm extends from the Sgr centernorthward of the Galactic plane to an ~40 kpc apoGalacticon, loops towards theNorth Galactic Cap (NGC) and descends back towards the Galactic plane,foreshortened and covering the NGC. The Sgr tails lie along a well-definedorbital plane that shows little precession, which supports the notion of anearly spherical Galactic potential. The Sun lies near the path of leading Sgrdebris; thus, former Sgr stars may be near or in the solar neighborhood. Thenumber of M giants in the Sgr tails is >15% that within the King limitingradius of the Sgr center. That several gigayear old M giants are so widespreadalong the Sgr tidal arms not only places limits on the dynamical age of thesearms but poses a timing problem that bears on the recent binding energy of theSgr core and that is naturally explained by recent and catastrophic mass loss.Sgr appears to contribute >75% of the high latitude, halo M giants; no evidencefor M giant tidal debris from the Magellanic Clouds is found. Generally goodcorrespondence is found between the M giant, all-sky map of the Sgr system andall previously published detections of potential Sgr debris with the exceptionof Sgr carbon stars -- which must be subluminous to resolve the discrepancy.