A tidal extension in the Ursa Minor dwarf spheroidal galaxy

We report the detection of main-sequence and blue horizontal-branch stars o f the Ursa Minor dwarf spheroidal galaxy beyond its tidal radius, indicatin g the existence of a possible tidal extension in this satellite of the Milk y Way. This tidal extension could spread out well beyond the area covered i n our survey (R > 80'), as suggested by the presence of a "break" to a shal lower slope observed in its density profile. The V-band surface brightness for this possible tidal extension ranges from 29.8 to 31.5 mag arcsec(-2). The area covered in our survey (similar to1.65 deg(2)) is not enough to dis criminate if this extratidal population is part of a tidal tail or an exten ded halo around the galaxy. The existence of this tidal extension in Ursa M inor indicates that this satellite is currently undergoing a tidal disrupti on process by the Milky Way. We discuss the possibility of a tidal origin f or the high mass-to-light ratio observed in this galaxy on the basis of our result and recent theoretical simulations of the tidal disruption of dwarf satellites in the Galactic halo.