On the Inclinations of the Host Galaxies of H₂O Masers

The H₂O maser emission has been detected so far in 104 external galaxies.Comparing to other galaxies, these host galaxies of H₂O masers may have some special properties not yet to be noticed.Here we provide the entire sample of identified galaxies with H₂O masers as compiled up to 2008.09.We calculate the inclinations of the stellar disks of these galaxies from observational data archives and discuss whether there is a relation between the orientations of the stellar disks and the accretion disks generating masers.For the orientations of the galaxy stellar disks, our results show that only 10% of galaxies with maser disks are nearly edge-on, i.e., the inclinations of the stellar disks are larger than 80°.However, Very Long Baseline Interferometry(VLBI)observations of six galaxies with maser disks show that the maser disks are almost edge-on.The statistical misalignment between accretion disks generating masers and stellar disks indirectly supports that disk masers are associated only with the galactic nuclei.We further investigate whether there are relations between maser luminosity, X-ray absorbing column density, and galaxy inclination in order to discuss the nature of obscuring material and possible mechanisms of maser amplification.Analysis results show that maser amplification most likely happens in the torus(~100pc)or in the even smaller-scale nuclear disk, which is the main location of obscuring material.