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 deg2) photometric and spectroscopic survey of the Leo I dwarf spheroidal (dSph) galaxy to explore its extended morphology and dynamics. To select Leo I red giant branch star candidates we exploit M, T2, and DDO51 filter photometry; this yields 100% pure Leo I stars among more than 100 M < 21.5 Leo I giant candidates having previous or new Keck spectroscopy. The two-dimensional distribution of all photometric Leo I giant candidates is well fitted by a single-component King profile of limiting radius 13.4' out to a major axis radial distance of ~10', but beyond this point the density profile shows an excess of stars along the major axis of the main body. This spatial configuration, together with a rather flat velocity dispersion profile and an asymmetric radial velocity distribution among Leo I members at large radii, supports a picture where Leo I has been tidally disrupted on one or two perigalactic passages about a massive Local Group member. We demonstrate this hypothesis using mass-follows-light, N-body simulations of satellites in a Milky Way-like potential that reproduce the observed structural and dynamical properties of Leo I remarkably well. These models include ~3 × 107 solar mass, tidally disrupting dSphs on bound orbits with rather high eccentricity (0.93-0.96) and small perigalactica (10-15 kpc). The simulations yield an observationally constrained orbit for Leo I without the measurement of its proper motion. Given the overall success of our satellite models to account for the observed properties of Leo I, we conclude that there is no need to invoke an extended dark matter halo around the satellite and that an overall modest M/L for the satellite is consistent with the available data.