Radiative Cooling and Viscous Dissipation in Molecular Accretion Disks at the Nuclei of Galaxies

Key processes are examined in the heating and cooling of molecular gas with emphasis on environments for water masers at the nuclei of galaxies. The exchange of infrared radiation between the gas and dust grains is evaluated in determining the cooling of the gas and in determining the thermal coupling between the gas and the dust grains. Calculations are performed for the transport of continuum radiation through an optically thick slab to assess the temperatures of the dust grains. The highly refined observational description of the subparsec circumnuclear disk in the galaxy NGC 4258 serves as a focus. The viscous dissipation that is inherent in an accretion disk is reconsidered as a source of heat to create the masing environment. Based on our cooling rates, the input of energy that is required to create the bulk of the observed masing gas is evaluated and is found to be comparable in magnitude (per unit surface area) to that generated by the viscous dissipation. To be effective in actually creating the masing environment, the distribution for the viscous dissipation of heat with distance from the midplane of the disk must be quite different from the distribution of the mass. Tentative evidence that this is plausible is discussed. One consequence is the likely presence of a component of the disk at the midplane that is colder and much more massive than the masing gas. The recent interpretation that the inner disk of NGC 4258 may be advective is especially useful for this description, since the viscous dissipation at the location of the masers tends to be enhanced. A significant temperature difference between the gas and the grains, which is necessary for masing, is found to result regardless of whether the heating in these environments is provided by viscous dissipation or by X-rays.