The IC 133 Water Vapor Maser in the Galaxy M33: A Geometric Distance

We report on the results of a 14 yr long VLBI study of proper motions in the IC 133 H₂O maser source in the galaxy M33. The method of ordered motion parallax was used to model the three-dimensional structure and dynamics of IC 133 and to obtain a distance estimate, 800+/-180 kpc. Our technique for determining the distance to M33 is independent of calibrations common to other distance indicators, such as Cepheid period-luminosity relations, and therefore provides an important check for previous distance determinations.