Megamaser Disks in Active Galactic Nuclei

Recent spectroscopic and VLBI-imaging observations of bright extragalactic HO maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed flux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show that even two clouds that overlap within the velocity-coherence length along the line of sight could account (through self-amplification) for the entire maser flux of a high-velocity "satellite" feature in sources like NGC 4258 and NGC 1068, and we suggest that cloud self-amplification likely contributes also to the flux of the background-amplifying "systemic" features in these objects. Analogous interpretations have previously been proposed for water maser sources in Galactic star-forming regions. We argue that this picture provides a natural explanation of the time-variability characteristics of extragalactic megamaser sources and of their apparent association with Seyfert 2-like galaxies. We also show that the requisite cloud space densities and internal densities are consistent with the typical values of nuclear (broad emission line region type) clouds. We examine two scenarios of clumpy disks in which the maser emission is excited by a central continuum source. This excitation mechanism was first considered in the context of megamaser disks by Neufeld & Maloney, but our proposed models are clearly distinct from their warped, homogeneous disk interpretation. In our first scenario we consider an annular disk (or "ring") whose inner edge corresponds to the innermost radius of the observed maser distribution and whose mass is dominated by the clumped, high-density gas component. The shielding of the high-energy continuum, which is required in order that the gas remain molecular, can be provided in this case by the dusty clouds themselves. We show that even the simplest version of this model, in which the disk is flat and the continuum radiation reaches the masing clouds through the plane of the disk, can account for the maser observations in NGC 1068. We point out the striking similarities between the maser ring properties as interpreted with this model and the inferred characteristics of the circumnuclear disk in the Galactic center, and we briefly discuss the implications of such rings for the AGN accretion disk paradigm. Our second scenario is motivated by the apparent warps observed in some of the imaged megamaser disks and by our finding that the flat-disk version of the irradiated ring scenario could apply to a source like NGC 4258 only if the water abundance in the masing clouds were higher than the value implied by equilibrium photoionization-driven chemistry. This scenario is based on the disk-driven hydromagnetic wind model originally proposed to account for the molecular "tori" in Seyfert 2-like galaxies and for several other observed phenomena in AGNs. In this picture, the wind uplifts (by its ram pressure) and confines (by its magnetic pressure) dense clouds fragmented from the disk, which mase after they become exposed to the central radiation field. Much of the requisite continuum shielding can be provided in this case by the dusty portions of the wind. We show that comparatively massive clouds that move in low-altitude, nearly circular orbits could be shielded in this way, and we suggest that an apparent warp in the maser distribution might arise under these circumstances from an observational selection effect induced by the strong vertical density stratification that characterizes a centrifugally driven wind. We construct a self-contained illustrative model where the wind transports the bulk of the disk angular momentum, and we show that it is consistent with the data for NGC 4258 as well as with the advection-dominated accretion flow interpretation of the spectrum in this source.