What Determines the Boundaries of H2O Maser Emission in an X-ray Illuminated Gas Disk ?
High precision mapping of H2O megamaser emission from active galaxies has revealed more than a dozen Keplerian H2O maser disks, which enable a ~4% uncertainty estimate of the Hubble constant as well as providing accurate masses for the central black holes. These disks often have well-defined inner and outer boundaries of maser emission on sub-parsec scales. In order to better understand the physical conditions that determine the inner and outer radii of a maser disk, we examine the distributions of gas density and X-ray heating rate in a warped molecular disk described by a power-law surface density profile. For a suitable choice of the disk mass, we find that the outer radius R_out of the maser disk predicted from our model can match the observed value, with R_out mainly determined by the maximum heating rate or the minimum density for efficient maser action, depending on the combination of the Eddington ratio, black hole mass, and disk mass. Our analysis also indicates that the inner radius for maser action is comparable to the dust sublimation radius, suggesting that dust may play a role in determining the inner radius of a maser disk. Finally, our model predicts that H2O gigamaser disks could exist at the centers of high-z quasars, with disk sizes of >~ 10-30 pc.