New insights on the Draco dwarf spheroidal galaxy from the Sloan Digital sky Survey: A larger radius and no tidal tails

We have investigated the spatial extent and structure of the Draco dwarf sp heroidal galaxy by using deep wide-field multicolor CCD photometry from the Sloan Digital Sky Survey (SDSS). Our study covers an area of 27 deg(2) aro und the center of the Draco dwarf and reaches 2 mag below the level of Drac o's horizontal branch. The SDSS photometry allows very effective filtering in color-magnitude space. With such filtering the density of the foreground of Galactic field stars is decreased by more than an order of magnitude, a nd the stellar population of the Draco dwarf galaxy thus stands out with mu ch higher contrast than in former investigations. We show that the spatial distribution of Draco's red giants, red horizontal-branch stars, and subgia nts down to i* = 21.7 mag does not provide evidence for the existence of ti dally induced tails or a halo of unbound stars. The projected surface densi ty of the dwarf galaxy is flattened with a nearly constant ellipticity of 0 .29 +/- 0.02 at position angle 88 degrees +/- 3 degrees. The radial profile can be fitted by King models, as well as by a generalized exponential. Usi ng the empirical King profile, the core radius and the limiting (or tidal) radius along the major axis are r(c) = 7.'7 and r(t) = 40.'1, respectively; the latter means that the size of the Draco dwarf galaxy is 40% larger tha n previously measured. Fitting the profile of King's theoretical models yie lds a still larger limiting radius of r(t) = 49.'5. There is no clear indic ation of a taillike extension of the Draco population beyond this radius. A break in the radial surface density profile, which might indicate a halo o f extratidal stars, is also not found in our Draco data. We conclude that d own to the above magnitude limit tidal effects can exist only at a level of 10(-3) of the central surface density of Draco or below. The regular struc ture of Draco found from the new data argues against its being a portion of an unbound tidal stream and lends support to the assumption of dynamical e quilibrium, which is the basis for mass estimation. The changes in the valu es for the core radius and limiting radius imply that the total mass of Dra co is higher by more than a factor of 2. Using a King spherical model of eq uivalent size as a reference and adopting a line-of-sight velocity dispersi on of either 10.7 or 8.5 km s(-1), we derive estimates of the total mass wi thin radius of (3.5 +/- 0.7) x 10(7) M. and (2.2 +/- 0.5) x 10(7) M., respe ctively. From the combined i-band flux of all possible Draco members that l ie within major-axis radius rt we determine the total luminosity of the Dra co dwarf galaxy as (L/L.)(i) = (2.4 +/- 0.5) x 10(5). This includes correct ions for the flux of the foreground stars and the unseen fainter part of th e Draco population. We thus obtain overall mass-to-light ratios of M/L-i of 146 +/- 42, or 92 +/- 28 in solar units. In summary, our results strengthe n the case for a strongly dark matter-dominated, bound stellar system.