Radio properties of H2O maser host galaxies

The 6 cm and 20 cm radio continuum properties of all 85 galaxies with reported 22 GHz H2O maser emission and luminosity distance D > 0.5 Mpc are studied. For the total of 55 targets for which both 6 cm and 20 cm measurements exist and for the subsample of 42 sources with masers related to active galactic nuclei (AGN), a spectral index could be determined from an assumed power-law dependence. The mean value of the resulting spectral index is in both cases 0.66 +/- 0.07 (S proportional to v(-alpha); S : flux density, v: frequency). Comparing radio properties of the maser galaxies with a sample of Seyferts without detected H2O maser, we find that (1) the spectral indices agree within the error limits; and (2) maser host galaxies have higher nuclear radio continuum luminosities, exceeding those of the comparison sample by factors of order 5. Only considering the subsample of galaxies with masers associated with AGN, there seems to be a trend toward rising maser luminosity with nuclear radio luminosity (both at 6 cm and 20 cm). However, when accounting for the Malmquist effect, the correlation weakens to a level, which is barely significant. Overall, the study indicates that nuclear radio luminosity is a suitable indicator to guide future AGN maser searches and to enhance detection rates, which are otherwise quite low (< 10%).