New insights on the Draco dwarf spheroidal galaxy from SDSS: a larger radius and no tidal tails

Michael Odenkirchen, Eva K. Grebel, Daniel Harbeck, Walter Dehnen, Hans-Walter Rix, et 18 alii from the SDSS collaboration

Published 2001-08-06Version 1

We have investigated the spatial extent and structure of the Draco dwarf spheroidal galaxy using deep wide-field multicolor CCD photometry from the Sloan Digital Sky Survey (SDSS). Our study covers an area of 27 square degrees around the center of the Draco dwarf. We show that the spatial distribution of Draco's red giants, red horizontal branch stars and subgiants down to i=21.7 mag does not provide evidence for the existence of tidally induced tails or a halo of unbound stars. The radial profile can be fit by King models as well as by a generalized exponential. The core radius and the limiting (or tidal) radius along the major axis are 7.7' and 40.1', respectively, making Draco 40% larger than previously measured. Down to our magnitude limit tidal effects can only exist at a level of 10^-3 of the central surface density of Draco or below. The regular structure of Draco found from the new data argues against it being a portion of an unbound tidal stream and lends support to the assumption of dynamical equilibrium. We estimate Draco's total mass to be 2.2 - 3.5 times 10^7 solar masses. We obtain an overall mass-to-light ratio of 146+-42 or 92+-28 solar masses depending on the details of the mass and luminosity estimates. In summary, our results strengthen the case for a strongly dark matter dominated, bound stellar system. (Abstract strongly abridged).