Infall Times for Milky Way Satellites From Their Present-Day Kinematics
  We analyze subhalos in the Via Lactea II (VL2) cosmological simulation to
look for correlations among their infall times and z = 0 dynamical properties.
We find that the present day orbital energy is tightly correlated with the time
at which subhalos last crossed into the virial radius. This energy-infall
correlation provides a means to infer infall times for Milky Way satellite
galaxies. Assuming that the Milky Way's assembly can be modeled by VL2, we show
that the infall times of some satellites are well constrained given only their
Galactocentric positions and line-of-sight velocities. The constraints sharpen
for satellites with proper motion measurements. We find that Carina, Ursa
Minor, and Sculptor were all accreted early, more than 8 Gyr ago. Five other
dwarfs, including Sextans and Segue 1, are also probable early accreters,
though with larger uncertainties. On the other extreme, Leo T is just falling
into the Milky Way for the first time while Leo I fell in \sim 2 Gyr ago and is
now climbing out of the Milky Way's potential after its first perigalacticon.
The energies of several other dwarfs, including Fornax and Hercules, point to
intermediate infall times, 2 - 8 Gyr ago. We compare our infall time estimates
to published star formation histories and find hints of a dichotomy between
ultrafaint and classical dwarfs. The classical dwarfs appear to have quenched
star formation after infall but the ultrafaint dwarfs tend to be quenched long
before infall, at least for the cases in which our uncertainties allow us to
discern differences. Our analysis suggests that the Large Magellanic Cloud
crossed inside the Milky Way virial radius recently, within the last \sim 4
billion years.
