H2O masers and the cosmic distance scale

VLBI proper-motion studies of H2O maser sources have revealed energetic expanding flows and provided a means to determine distances across the Galaxy. These studies measure the relative positions of several tens of maser spots in one source over epochs typically spanning months to years. Ground-based VLBI observations may be extended to H2O masers in galaxies less than about 1 Mpc away. The increased angular resolution and dynamic range afforded by space-based VLBI may allow measurement of distances to galaxies out to about 10 Mpc. 8 references