The Velocity Dispersion Profile of the Remote Dwarf Spheroidal Galaxy Leo I: A Tidal Hit and Run?

Abstract

We present new kinematic results for a sample of 387 stars located in and around the Milky Way satellite dwarf spheroidal galaxy Leo I. These spectra were obtained with the Hectochelle multi-object echelle spectrograph on the MMT, and cover the MgI/Mgb lines at about 5200˚A. Based on 297 repeat measurements of 108 stars, we estimate the mean velocity error (1σ) of our sample to be 2.4 km/s, with a systematic error of ≤ 1 km/s. Combined with earlier results, we identify a final sample of 328 Leo I red giant members, from which we measure a mean heliocentric radial velocity of 282.9 ± 0.5 km/s, and a mean radial velocity dispersion of 9.2 ± 0.4 km/s for Leo I. The dispersion profile of Leo I is essentially flat from the center of the galaxy to beyond its classical ‘tidal’ radius, a result that is unaffected by contamination from field stars or binaries within our kinematic sample. We have fit the profile to a variety of equilibrium dynamical models and can strongly rule out models where mass follows light. Two-component Sersic+NFW models with tangentially anisotropic velocity distributions fit the dispersion profile well, with isotropic models ruled out at a 95% confidence level. Within the projected radius sampled by our data ( ∼ 1040 pc), the mass and V-band mass-to-light ratio of Leo I estimated from equilibrium models are in the ranges 5-7 ×10 7 M ⊙ and 9-14 (solar units), respectively. We find that Leo I members located outside a ‘break radius ’ at Rb ∼ 400 arcsec (500 pc) exhibit significant velocity anisotropy, whereas stars interior of this radius appear consistent with an isotropic velocity distribution. We propose a heuristic model in which the break radius represents the location of the tidal radius of Leo I at perigalacticon of a highly elliptical orbit. Our scenario can account for