Strangers in the Night: Discovery of a Dwarf Spheroidal Galaxy on Its First Local Group Infall

We present spectroscopic observations of the And XII dwarf spheroidal galaxy using DEIMOS/Keck II, showing it to be moving rapidly through the Local Group (-556 km s -1 heliocentric velocity, -281 km s -1 relative to Andromeda), falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. And XII therefore represents a dwarf galaxy plausibly falling into the Local Group for the first time and never having experienced a dense galactic environment. From Green Bank Telescope observations, a limit on the H I gas mass of <3 × 10 3 M ☉ suggests that And XII's gas could have been removed prior to experiencing the tides of the Local Group galaxies. Orbit models suggest that the dwarf is close to the escape velocity of M31 for published mass models. And XII is our best direct evidence for the late infall of satellite galaxies, a prediction of cosmological simulations.