Megamasers and Galaxies

Fred Lo
Institute of Astronomy & Astrophysics
Academia Sinica

Abstract: 

While masers on Earth required invention in the 50's, OH and H_2O maser
emissions are commonly found in high density and high temperature gas near
young stellar objects or evolved stars in our Galaxy.  Very powerful 
extragalactic OH and H_2O masers (mega-masers) are
found in nuclei of galaxies, in the molecular media in the
immediate vicinity of AGN's and in star-burst regions.  In particular,
high resolution imaging of the H_2O mega-maser in NGC 4258 led to the
first direct delineation of a thin Keplerian accretion disk around a
massive compact object. Because of their high luminosity and brightness,
mega-masers are important for sub-parsec to parsec scales probes of the
molecular media around AGN's and star-burst regions at large distances.
They could also be very useful probes of young galaxies at high
redshifts