Draco, a flawless dwarf galaxy

Abstract

The Draco dwarf spheroidal galaxy (dSph), with its apparent immense mass to light ratio and compact size, holds many clues to the nature of the enigmatic dark matter. Here we present deep photometric studies of this dwarf galaxy, undertaken with the MegaCam Camera at the Canada-France-hawaii Telescope, the Wide Field Camera at the Isaac Newton Telescope and the Wide-Field and Planetary Camera on board the Hubble Space Telescope. The new photometric data cover the entirety of the galaxy, and reach i=24.5 at 50% completeness, significantly deeper than previous panoramic studies, allowing searches for tidal disturbances of much lower surface brightness than has been possible before. With these improved statistics, we find no evidence for asymmetric disturbances or tidal tails that possess more than 3% of the stars found within the centre of the galaxy. We find that the central stellar density, as probed by the HST data, rises into the central 0.5'. Uncertainties in the position of the centroid of the galaxy do not allow us to determine whether the apparent flattening of the profile interior to 0.5' is reliable or not. Draco is therefore a flawless dwarf galaxy, featureless and apparently unaffected by Galactic tides.