Exploring Halo Substructure with Giant Stars. X. Extended Dark Matter or Tidal Disruption?: The Case for the Leo I Dwarf Spheroidal Galaxy

We present a wide-field (4.5 deg^2) photometric and spectroscopic survey ofthe Leo I dwarf spheroidal (dSph) galaxy to explore its extended morphology anddynamics. As in previous papers in this series, we take advantage of photometryin the M, T_2, and DDO51 filter system to select LeoI red giant branch starcandidates, and, so far, this selection technique has proven 100% reliable inselecting actual Leo I members among more than 100 M < 21.5 Leo I giantcandidates having previous or new Keck DEIMOS spectroscopy to a radius >1.3times the limiting radius of the fitted, central King profile. Thetwo-dimensional distribution of all similarly-selected Leo I giant candidatesis well fitted by a central single-component King profile of limiting radius13.3 arcmin, but many giant stars are found outside this newly derived Kinglimiting radius. The density profile thus shows a break at a major axis radialdistance of ~10 arcmin produced by an excess of stars at and beyond the Kinglimiting radius (spectroscopically confirmed to be made of true Leo I members),and primarily along the major axis of the main body of the rather elongatedsatellite. This spatial configuration, a rather flat velocity dispersionprofile and an asymmetric radial velocity (RV) distribution among the Leo Imembers at large radii together support a picture where Leo I has been tidallydisrupted on at least one, but at most two, perigalactic passages of a massiveLocal Group member. (abridged)Comment: Accepted by Astrophysical Journal on March 15, 2007; 49 pages, 26 figures, uses emulateapj.cl