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

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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 ∼10 years. We obtain a motion of μ α cos(δ) =-0.37 ±0.57 mas yr-1 and μ δ = -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 km s-1. Applying Milky Way halo masses of between 0.8 and 1.6 ×1012 M o results in an apocenter at kpc and pericenter at 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 Gyr ago, a median of four pericentric passages since then, and a pericenter of 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.

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