Title: The Impact of Masers in Modern Astrophysics
by James Moran （Donald H. Menzel Professor of Astrophysics, Emeritus）
Harvard-Smithsonian Center for Astrophysics
Prof. James Moran's home page: https://www.cfa.harvard.edu/~jmoran/
Abstract: I will describe the discovery of cosmic masers in 1965 and their subsequent use as probes of astrophysical phenomena. I will concentrate on two topics: the measurement of magnetic field strength in regions of star formation and the measurements of the masses and distances to black holes in the centers of active galaxies through the Keplerian motions of their associated water vapor masers. The archetypical maser of this type is associated with the galaxy NGC4258. It was discovered in 1984 by Fred Lo and his collaborators at the Owens Valley Radio Observatory. Their work transformed the paradigm for extragalactic maser emission. I will describe how precise measurement of the masers in this galaxy have led to very accurate determinations of the mass of the associated black hole (3.9 +/- 0.1 x 10^7 solar masses) and its distance (7.5 +/- 0.2 Mpc). The masers in NGC4258 have been detected on baselines of up to 26 Earth diameters (a resolution of 8 microarcseconds) with the RadioAstron mission. I will also discuss the impact of maser distance measurements on the determination of H_o, a topic of particular interest to Lo.