http://arxiv.org/abs/1711.09097
We present a proper motion measurement for the ultra-faint dwarf spheroidal galaxy Segue 1, using SDSS and LBC data as the first and second epochs, separated by a baseline of ~10 years. This is not only the first proper motion reported for Segue 1, but also the first for any ultra-faint dwarf galaxy. We obtain a measurement of mu_alpha * cos(delta) =-0.37+/-0.57 mas/yr and mu_delta=-3.39+/-0.58 mas/yr. Combining this with the known line-of-sight velocity, this corresponds to Galactocentric V_rad=84+/-18 and V_tan=172$^{+74}{-66}$ km/s. In conjunction with the Milky Way halo masses between 0.8 to 1.6*10^12 M_sun, this results in an apocenter at 35.0$^{+22.3}{-6.9}$ kpc and pericenter at $15.8^{+9.4}{-8.3}$ kpc from the Galactic center, indicating Segue 1 is rather tightly bound to the Milky Way. The orbit of Segue 1 is not similar to any of the orbits of the more massive classical dwarf galaxies with published proper motions, and, most significantly, has a different orbital pole. Further, the other dwarf galaxies spend large parts of their orbits at greater distances from the Milky Way than Segue 1, making it 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 9.4$^{+2.2}{-4.5}$ Gyrs ago, a median of 5 pericentric passages since then, and a pericenter of 23.5$^{+4.8}_{-5.3}$ 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 by the Galaxy. 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.
T. Fritz, M. Lokken, N. Kallivayalil, et. al.
Tue, 28 Nov 17
70/82
Comments: 16 pages, 7 figures, submitted to ApJ