Draco: A Failure of the Tidal Model

Abstract

We test whether the structural properties of the nearby dwarf spheroidal (dSph) galaxy Draco, a well-studied Milky Way companion, can be reconciled with the suggestion that dSphs are unbound tidal remnants with a large depth extent along the line of sight. In order to apply the observational test of this hypothesis, suggested by Klessen & Zhao, we use public photometric data from the Sloan Digital Sky Survey (SDSS) to explore the width of Draco's blue horizontal branch over a range of areas covering 0.06-6.25 deg2 centered on Draco. The SDSS database is the only currently existing database with sufficient depth and area coverage to permit a stringent test of the tidal models. Blue horizontal branch stars were chosen as tracers of Draco's spatial extent and depth because of their low contamination by Galactic foreground stars and since they have a spatially more extended distribution than the more centrally concentrated red horizontal branch stars. Indeed, we show that blue horizontal branch stars extend beyond the previously inferred limiting radii of Draco, consistent with the observed absence of a truncated stellar surface density profile of this dSph (Odenkirchen et al.). Following the method of Klessen & Kroupa, we calculate new models for a galaxy without dark matter, using Draco's morphological properties as constraints. The resulting models are unable to reproduce the narrow observed horizontal branch width of Draco, which stays roughly constant regardless of the field of view. We conclude that Draco cannot be the remnant of a tidally disrupted satellite but is probably strongly dark matter-dominated, as suggested independently by the structural analysis conducted by Odenkirchen et al. and by the kinematic analysis of Kleyna et al.