The Proper Motion of the Local Group Galaxy IC 10

We have measured the proper motion of the Local Group galaxy IC 10 with the Very Long Baseline Array by measuring the position of an H2O maser relative to two background quasars over 4.3 years. After correction for the rotation of the Milky Way and IC 10, we obtain a motion of -39 +/- 9 microas/yr toward the East and 31 +/- 8 microas/yr toward the North. This corresponds to a total space velocity of 215 +/- 42 km/s relative to the Milky Way for an assumed distance of 660 +/- 66 kpc. Assuming that IC 10 and M33, for which also a proper motion measurement exists, are bound to M31, we calculate a lower limit for the mass of M31 of 7.5 x 10^11 Msol.