Abstract
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.