Draco: A Failure of the Tidal Model

We test whether the structural properties of the nearby dwarf spheroidal(dSph) galaxy Draco, a well-studied Milky Way companion, can be reconciled withthe suggestion that dSphs are unbound tidal remnants with a large depth extentalong the line of sight. In order to apply the observational test of thishypothesis suggested by Klessen & Zhao, we use public photometric data from theSloan Digital Sky Survey (SDSS) to explore the width of Draco's blue horizontalbranch over a range of areas covering 0.06 square degrees to 6.25 squaredegrees centered on Draco. The SDSS database is the only currently existingdatabase with sufficient depth and area coverage to permit a stringent test ofthe tidal models. We show that blue horizontal branch stars extend beyond thepreviously inferred limiting radii of Draco, consistent with the observedabsence of a truncated stellar surface density profile of this dSph. Wecalculate new models for a galaxy without dark matter, using Draco'smorphological properties as constraints. The resulting models are unable toreproduce the narrow observed horizontal branch width of Draco, which staysroughly constant regardless of the field of view. We conclude that Draco cannotbe the remnant of a tidally disrupted satellite, but is probably stronglydark-matter dominated. (ABSTRACT ABBREVIATED)