Exploring Halo Substructure with Giant Stars. IV. The Extended Structure of the Ursa Minor Dwarf Spheroidal Galaxy

Abstract

We present a large-area photometric survey of the Ursa Minor dwarf spheroidal galaxy and its environs. This survey is intended to trace the distribution of stars outside the nominal tidal radius of this system. Observations were made with the Washington M, Washington T 2 , and DDO51 filters, which in combination have been shown to provide reliable stellar luminosity classification for K-type stars. We identify giant star candidates with the same distance and metallicity as known Ursa Minor red giant branch stars extending to approximately 3° from the center of the dSph. Comparison with catalogs of stars within the tidal radius of Ursa Minor that have been observed spectroscopically suggests that our photometric luminosity classification is 100% accurate. Over a large fraction of the survey area, our photometry is deep enough that blue horizontal branch stars associated with Ursa Minor can also be identified. The spatial distribution of both the candidate Ursa Minor giant stars and the candidate blue horizontal branch stars are remarkably similar, and for both samples a large fraction of the stars are found outside the nominal tidal radius of Ursa Minor. An isodensity contour map of the surface density of stars within the tidal radius of Ursa Minor reveals several morphological peculiarities: (1) The highest density of dSph stars is not found at the center of symmetry of the outer isodensity contours but instead is offset southwest of center. (2) The overall shape of the outer contours does not appear to be elliptical, but appears S-shaped. A surface density profile was derived for Ursa Minor and compared with those derived from previous studies. We find that previously determined King profiles with ∼50′ tidal radii do not fit well the distribution of candidate UMi stars identified in this study, which extends to greater radii than these other surveys. A King profile with a much larger tidal radius produces a reasonable fit; however, a power law with index -3 provides an even better fit to the densities at radii greater than 20′. The existence of stars associated with Ursa Minor at large distances from the core of the galaxy, the peculiar morphology of the galaxy within its tidal radius, and the shape of its surface density profile all suggest that this system is evolving significantly because of the tidal influence of the Milky Way. However, the photometric data on Ursa Minor stars alone do not allow us to determine if the candidate extratidal stars are now unbound or if they remain bound to the dSph within an extended dark matter halo.