Having conducted a search for the \lambda \sim 1.3 cm ( 22 GHz ) water vapor line towards galaxies with nuclear activity , large nuclear column densities or high infrared luminosities , we present H _ { 2 } O spectra for NGC 2273 , UGC 5101 , and NGC 3393 with isotropic luminosities of 7 , 1500 , and 400 L _ { \odot } .
The H _ { 2 } O 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 N _ { H } \ga 10 ^ { 24 } cm ^ { -2 } .
For NGC 5728 , partial absorption with a power law spectrum indicates N _ { H } \sim 8 \times 10 ^ { 23 } cm ^ { -2 } .
The correlation between absorbing column and H _ { 2 } O emission is analyzed .
There is a striking difference between kilo- and megamasers with megamasers being associated with higher column densities .
All kilomasers ( L _ { H _ { 2 } O } < 10 L _ { \odot } ) except NGC 2273 and NGC 5194 are Compton-thin , i.e .
their absorbing columns are < 10 ^ { 24 } cm ^ { -2 } .
Among the H _ { 2 } O megamasers , 50 % arise from Compton-thick and 85 % from heavily obscured ( > 10 ^ { 23 } 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 .