Microarcsecond proper motions of extragalactic water vapor masers in M33

We present a second-epoch spectral line VLBI synthesis map of the H_2_ O maser associated with the H II region IC 133 in the galaxy M33. This map is about 2.5 times more sensitive than the earlier one. Thirty- two spatially distinct maser features have been identified, and we have discovered a second center of maser activity within the IC 133 complex, IC 133 West, which is displaced ~0.3" (1 pc at a nominal distance of 720 kpc) from IC 133 Main. The most blueshifted maser features are located in IC 133 West, which may mark the location of a separate star forming region. Comparing the two available maps of IC 133, we have estimated the right ascension components of proper motion over a period of 479 days for five maser features to accuracies of between 7 and 16 microas. The dispersions in transverse and radial velocities for the maser features are consistent with the accepted distance to M33 of 720 kpc, where the data admit a fractional uncertainty in distance of 50%. Our result represents the first successful measurements of proper motions in an extragalactic H_2_O maser source and demonstrate the feasibility of making such measurements in other galaxies. Improvements in this technique may make it important for the estimation of extragalactic distances.