The Orbit and Origin of the Ultra-faint Dwarf Galaxy Segue 1

We present the first proper motion measurement for an ultra-faint dwarf spheroidal galaxy, Segue 1, using data from the Sloan Digital Sky Survey (SDSS) and the Large Binocular Camera (LBC) as the first and second epochs separated by a baseline of similar to 10 years. We obtain a motion of mu(alpha) cos(delta) = -0.37 +/- 0.57mas yr(-1) and mu(delta) = -3.39 +/- 0.58 mas yr(-1). Combining this with the known line-of-sight velocity, this corresponds to a Galactocentric V-rad = 84 +/- 9 and V-tan 164(-55)(+66) km s(-1) Applying Milky Way halo masses of between 0.8 and 1.6 x 10(12)M(circle dot) results in an apocenter at 33.9(-7.4)(+21.)7 kpc and pericenter at 15.4(-9.0)(+10.1) kpc from the Galactic center, indicating that Segue. 1 is rather tightly bound to the Milky Way. Since neither the orbital pole of Segue. 1 nor its distance to the Milky Way is similar to the more massive classical dwarfs, it is very unlikely that Segue 1 was once a satellite of a massive known galaxy. Using cosmological zoom-in simulations of Milky Way-mass galaxies, we identify subhalos on similar orbits as Segue. 1, which imply the following orbital properties: a median first infall 8.1(-4.3)(+3.6) Gyr ago, a median of four pericentric passages since then, and a pericenter of 22.8(-4.8)(+4.7) kpc. This is slightly larger than the pericenter derived directly from Segue 1 and Milky Way parameters, because galaxies with a small pericenter are more likely to be destroyed. Of the surviving subhalo analogs, only 27% were previously a satellite of a more massive dwarf galaxy (that is now destroyed), thus Segue 1 is more likely to have been accreted on its own.