Improved photometry of Sloan Digital Sky Survey crowded-field images: structure and dark matter content in the dwarf spheroidal galaxy Leo I

We explore how well crowded-field point-source photometry can be accomplished with Sloan Digital Sky Survey (SDSS) data. For this purpose, we present a photometric pipeline based on DoPHOT and tuned for analyzing crowded-field images from the SDSS. Using Monte Carlo simulations we show that the completeness of source extraction is above 80% to an i-band AB magnitude of <~21 and a stellar surface density of ~200 arcmin-2. Hence, a specialized data pipeline can be efficiently used for, e.g., nearby resolved galaxies in SDSS images, where the standard SDSS photometric package Photo, when applied in normal survey mode, gives poor results. We apply our pipeline to an area of ~3.55 deg2 around the dwarf spheroidal galaxy (dSph) Leo I and construct a high signal-to-noise ratio star count map of Leo I via an optimized filter in color-magnitude space (g, r, i). This filter reduces the foreground contamination by ~80% and enhances the central stellar surface density contrast of the dwarf by a factor of >~4, making this study the deepest wide-field study of the Leo I dSph based on accurate CCD photometry. Although the radial surface density profile deviates from the best-fit empirical King model toward outer radii, we find no evidence for tidal debris out to a stellar surface density of 4×10-3 of the central value. We determine the total luminosity of Leo I and model its mass using the spherical and isotropic Jeans equation. Assuming that mass follows light, we constrain a lower limit on the total mass of the dSph to be (1.7+/-0.2)×107 Msolar. On the other hand, if the mass in Leo I is dominated by a constant-density dark matter (DM) halo, then the mass within the central 12' is (2+/-0.6)×108 Msolar. This leads to a mass-to-light ratio of >>6 in I-band solar units, and possibly >75 if the DM halo dominates the mass and extends further out than 12'. In summary, our results show that Leo I is a symmetric, relaxed, and bound system; this supports the idea that Leo I is a DM-dominated system