Dense Molecular Gas and H₂O Maser Emission in Galaxies

Abstract

Extragalactic H₂O masers have been found in dense gas circumstance in off-nuclear star formation regions or within parsecs of Active Galactic Nuclei (AGNs). HCN molecular (one of the best dense gas tracers) emission has been detected in more than 60 galaxies. For HCN-detected galaxy sample, the relation of maser and gas emission was investigated here to identify physical observable properties that differentiate maser and non-maser galaxies. Our analysis results show that there is no significant difference on the infrared and gas emission between maser galaxies and galaxies without maser detection. For maser host HCN-galaxies, maser luminosity is found to be correlated to CO luminosity (a proxy of the total molecular gas) and HCN luminosity, i.e., kilomasers ($L_{\mathrm{H}_{2}\mathrm{O}}$ < 10 L _⊙) with low maser luminosity having low gas emission luminosity, with respect to megamasers ($L_{\mathrm{H}_{2}\mathrm{O}}$ > 10 L _⊙). For normalized maser and HCN luminosity (for removing distance effect), the correlation is still apparent. However, for normalized maser and CO luminosity, the correlation disappeared completely. Thus one proposition that the amount of dense molecular gas should be a good tracer of H₂O maser emission can be made.