We present the first absolute proper motion measurement of Leo I , based on two epochs of Hubble Space Telescope ( HST ) ACS/WFC images separated by \sim 5 years in time .
The average shift of Leo I stars with respect to \sim 100 background galaxies implies a proper motion of ( \mu _ { W } , \mu _ { N } ) = ( 0.1140 \pm 0.0295 , -0.1256 \pm 0.0293 ) { mas yr } ^ { -1 } .
The implied Galactocentric velocity vector , corrected for the reflex motion of the Sun , has radial and tangential components V _ { rad } = 167.9 \pm 2.8 \ > { km } { s } ^ { -1 } and V _ { tan } = 101.0 \pm 34.4 \ > { km } { s } ^ { -1 } , 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 and its surrounding galaxies .
Leo I entered the Milky Way virial radius 2.33 \pm 0.21 Gyr ago , most likely on its first infall .
It had a pericentric approach 1.05 \pm 0.09 Gyr ago at a Galactocentric distance of 91 \pm 36 kpc .
We associate these time scales with characteristic time scales in Leo I ’ s star formation history , which shows an enhanced star formation activity \sim 2 Gyr ago and quenching \sim 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 \sim 1 Gyr .
For most plausible Milky Way masses , the observed velocity implies that Leo I is bound to the Milky Way .
However , it may not be appropriate to include it in models of the Milky Way satellite population that assume dynamical equilibrium , given its recent infall .
Solution of the complete ( non-radial ) timing equations for the Leo I orbit implies a Milky Way mass M _ { MW,vir } = 3.15 _ { -1.36 } ^ { +1.58 } \times 10 ^ { 12 } { M _ { \odot } } , 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 .