1ST-IMAGES OF WATER-VAPOR MASERS IN THE GALAXY-M33

We present a spectral-line VLBI synthesis map of the H2O maser associated with the H II region complex IC 133 in the spiral galaxy M33. This is the first VLBI synthesis map of an H2O maser found in the spiral arm of another galaxy. The maser emission extends over ∼30 milliarcseconds (0.1 pc at a distance of 720 kpc) and is divided into 14 distinct spatial components, the strongest of which has a correlated flux density of ∼0.7 Jy. The relative positions of some of these components are accurately enough determined to provide first-epoch measurements for proper motion studies. The characteristics of the maser are similar to those of the most powerful maser in our own Galaxy, W49N. We have studied radio continuum emission in the immediate vicinity of the maser using the VLA and found a compact H II region close to the maser in position. This H II region is ∼1 pc in diameter, and its emission measure is about 6 × 107 pc cm-6. The H II region is the compact component of a more extended H II complex that extends over ∼100 pc. We have also detected weak fringes from a second maser in M33, M33/19, located ∼4.4 kpc in projection from IC 133. We searched for new extragalactic H2O masers by surveying 535 fields in 22 galaxies of the Local and M81 Groups but found none that is stronger than about 1 Jy.