Abstract
Having conducted a search for the λ ~ 1.3 cm (22 GHz) water vapor line towards galaxies with nuclear activity, large nuclear column densities or high infrared luminosities, we present H2O spectra for NGC 2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and 400 $L_{\odot}$. The H2O maser in UGC 5101 is by far the most luminous yet found in an ultraluminous infrared galaxy. NGC 3393 reveals the classic spectrum of a “disk maser”, represented by three distinct groups of Doppler components. As in all other known cases except NGC 4258, the rotation velocity of the putative masing disk is well below 1000 km s-1. Based on the literature and archive data, X-ray absorbing column densities are compiled for the 64 galaxies with reported maser sources beyond the Magellanic Clouds. For NGC 2782 and NGC 5728, we present Chandra archive data that indicate the presence of an active galactic nucleus in both galaxies. Modeling the hard nuclear X-ray emission, NGC 2782 is best fit by a high energy reflection spectrum with NH $\ga$ 1024 cm-2. For NGC 5728, partial absorption with a power law spectrum indicates NH ~ 8 $\times$ 1023 cm-2. The correlation between absorbing column and H2O emission is analyzed. There is a striking difference between kilo- and megamasers with megamasers being associated with higher column densities. All kilomasers ($L_{\rm H_2O}$ < 10 $L_{\odot}$) except NGC 2273 and NGC 5194 are Compton-thin, i.e. their absorbing columns are <1024 cm-2. Among the H2O megamasers, 50% arise from Compton-thick and 85% from heavily obscured (>1023 cm-2) active galactic nuclei. These values are not larger but consistent with those from samples of Seyfert 2 galaxies not selected on the basis of maser emission. The similarity in column densities can be explained by small deviations in position between maser spots and nuclear X-ray source and a high degree of clumpiness in the circumnuclear interstellar medium.