2013ApJ...768..139S

We present the first absolute proper motion measurement of Leo I, based on two epochs of Hubble Space Telescope ACS/WFC images separated by ∼5 years in time. The average shift of Leo I stars with respect to ∼100 background galaxies implies a proper motion of (µ_W, µ_N) = (0.1140±0.0295, -0.1256±0.0293) mas/yr. The implied Galactocentric velocity vector, corrected for the reflex motion of the Sun, has radial and tangential components V_rad= 167.9±2.8 km/s and V_tan= 101.0±34.4 km/s, respectively. We study the detailed orbital history of Leo I by solving its equations of motion backward in time for a range of plausible mass models for the Milky Way (MW) and its surrounding galaxies. Leo I entered the MW virial radius 2.33±0.21 Gyr ago, most likely on its first infall. It had a pericentric approach 1.05±0.09 Gyr ago at a Galactocentric distance of 91±36 kpc. We associate these timescales with characteristic timescales in Leo I's star formation history, which shows an enhanced star formation activity ∼2 Gyr ago and quenching ∼1 Gyr ago. There is no indication from our calculations that other galaxies have significantly influenced Leo I's orbit, although there is a small probability that it may have interacted with either Ursa Minor or Leo II within the last ∼1 Gyr. For most plausible MW masses, the observed velocity implies that Leo I is bound to the MW. However, it may not be appropriate to include it in models of the MW satellite population that assume dynamical equilibrium, given its recent infall. Solution of the complete (non-radial) timing equations for the Leo I orbit implies an MW mass M_MW,vir = 3.15_–1.36^+1.58 x 10² M_☉ , with the large uncertainty dominated by cosmic scatter. In a companion paper, we compare the new observations to the properties of Leo I subhalo analogs extracted from cosmological simulations.