Improved Photometry of Sdss 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 (Schechter et al. 1993), 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. Hence, a specialized data pipeline can be efficiently used for fairly crowded fields, such as 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 ∼ 3.552 around the dwarf spheroidal galaxy (dSph) Leo I. Using the resulting multi-band (g,̊,i) photometry we construct a high signal-to-noise star-count map of Leo I, utilizing an optimized filter in color-magnitude space. 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. We find that the projected spatial structure of Leo I is ellipsoidal. The best fitting empirical King model to the stellar-surface density profile yields core and tidal radii of (6.21± 0.95) and (11.70 ± 0.87), respectively. This corresponds to (460 ± 75) pc and (860 ± 86) pc assuming a distance to Leo I of 254 −16 kpc. The radial surface-density profile deviates from the King profile towards outer radii, yet we find no evidence for ’S’ shaped or irregular tidal debris out to a stellar surface-density of 4×10 of the central value. From the luminosity function of all possible Leo I stars, which we carefully extrapolated to faintest magnitudes, we determine the total Ic-band luminosity of Leo I to be (3.0 ± 0.3)×10 LIc,⊙. We model the mass of the dSph using the spherical and isotropic Jeans equation and infer a central mass density of 0.07M⊙ pc −3 leading to a central mass-to-light ratio of ∼ 3 in Ic band solar units. Assuming that the mass in the system follows the distribution of the visible component, we constrain a lower limit on the total mass of the dSph to be (1.7±0.2)×10 M⊙. On the other hand, if the mass in Leo I is dominated by a dark-matter (DM) halo with constant density, then the mass within the central 12 yields (2 ± 0.6)×10 M⊙. Combining the inferred mass estimates with the total luminosity leads to a mass-to-light ratio of ≫ 6 in Ic 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 dark-matter dominated system. Subject headings: surveys; galaxies: Local Group, dwarf, halos; cosmology: dark matter; methods: data analysis; techniques: image processing