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. As in previous papers in this series, we take advantage of photometry in the M, T2, and DDO51 filter system to select Leo I red giant branch star candidates, and, so far, this selection technique has proven 100% reliable in selecting actual Leo I members among more than 100 M < 21.5 Leo I giant candidates having previous or new Keck DEIMOS spectroscopy to a radius> 1.3 times the limiting radius of the fitted, central King profile. The two-dimensional distribution of all similarlyselected Leo I giant candidates is well fitted by a central single-component King profile of limiting radius 13.3 arcmin, but many giant stars are found outside this newly derived King limiting radius. The density profile thus shows a break at a major axis radial distance of ∼ 10 arcmin produced by an excess of stars at and beyond the King limiting radius (spectroscopically confirmed to be made of true Leo I members), and primarily along the major axis of the main body of the rather elongated satellite. This spatial configuration, a rather flat velocity dispersion profile and an asymmetric radial velocity (RV) distribution among the Leo I members at large radii together support a picture where Leo I has been tidally disrupted on at least one, but at most two, perigalactic passages of 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 dSph systems on bound orbits with rather high eccentricity (0.93–0.96) and small perigalactica (10-15 kpc). The simulations allow the first observationally constrained orbit for Leo I without the measurement of its proper motion and show that the observed RV distribution is more consistent with a two Milky Way orbit history for the satellite while ruling out a Leo I orbit that includes a previous association with M31 within the last 10 Gyr. Given the overall success of tidally disrupting mass-follows-light 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 (e.g., as one explanation of the velocity dispersion and radial profiles at large radii), and that an overall modest M/L for the satellite is consistent with the available data. That a satellite on such a large (apogalacticon of ∼ 450 kpc), long period (P ∼ 6 Gyr) orbit as Leo I can experience tidal disruption suggests that similarly structured satellites with even smaller (eccentric) orbits will experience even greater tidally-induced mass loss rates. Subject headings: galaxies:evolution — galaxies:interactions — galaxies:halos — galaxies:individual(Leo I) — galaxies:photometry — galaxies:structure 1 Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903 ([email protected]; srm4n, rrm8f, jco9w, rjp0i, [email protected]) 2 Korea Astronomy and Space Science Institute, 61-1, Hwaam-dong, Yuseong-gu, Daejeon 305-348, Korea ([email protected]) 3 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories (NOAO). NOAO is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. 4 Current address: California Institute of Technology, MC 405-47, 1200 East California Boulevard, Pasadena, CA 91125 ([email protected]) 5 Las Campanas Observatory, Casilla 601, La Serena, Chile ([email protected]) 6 Van Vleck Observatory, Wesleyan University, Middletown, CT 06459; current address: Department of Astronomy, Columbia University, New York, New York 10027 ([email protected]) 7 Current address: 1810 Kalorama Road NW, A3, Washington, DC 20009 ([email protected]) 8 UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064 ([email protected]) 9 Current address: Department of Astronomy, University of Texas, Austin, TX 78712 ([email protected]) 10 Department of Astronomy, University of California, Berkeley, CA 94720-3411 ([email protected])