The Warp in the Subparsec Molecular Disk in NGC 4258 as an Explanation for Persistent Asymmetries in the Maser Spectrum

Abstract

Neufeld & Maloney have proposed a model in which the warp in the subparsec-scale molecular disk in NGC 4258 allows direct illumination of the disk by a central X-ray source, heating the gas to temperatures appropriate for collisional pumping of water vapor masers. One element of this model is the creation of a layer of warm atomic gas above the cooler molecular gas where the masing occurs. Here, we examine the effects of this warm atomic gas on the maser emission. We find that for a range of plausible warps, thermal free-free extinction in the atomic gas can generate opacities of 2-3 at a wavelength of 1.3 cm. Furthermore, because of the orientation of the warp with respect to the line of sight, we argue that this extinction will only occur on the blueshifted side of the disk. Thus, we demonstrate that such thermal absorption is a likely explanation for the persistent weakness of the blueshifted maser features relative to the redshifted ones.