What determines the boundaries of H2O maser emission in an X-ray illuminated gas disc?

Abstract

ABSTRACT High precision mapping of H$_{2}$O megamaser emission from active galaxies has revealed more than a dozen Keplerian H$_{2}$O maser discs, which enable a $\sim$4 per cent uncertainty estimate of the Hubble constant as well as providing accurate masses for the central black holes. These discs 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 disc, we examine the distributions of gas density and X-ray heating rate in a warped molecular disc described by a power-law surface density profile. For a suitable choice of the disc mass, we find that the outer radius $R_{\rm out}$ of the maser disc predicted from our model can match the observed value, with $R_{\rm out}$ 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 disc 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 disc. Finally, our model predicts that H$_{2}$O gigamaser discs could exist at the centres of high-z quasars, with disc sizes of $\gtrsim 10-30$ pc.