H2O megamaser emission from FRI radio galaxies

A systematic search for 22 GHz H2O megamaser emission is reported for 50 nearby (z less than or similar to 0.15) FRI galaxies. No detection was obtained, implying that ultraluminous H2O masers (L-H2O > 10(3) L-.) must be rare in early-type galaxies with FR I radio morphology. Despite higher radio core luminosities the detection rate for our sample is lower than in similar surveys of late-type Seyfert galaxies. This puzzling difference between Seyferts and low-power radio galaxies could be explained in several ways: a) the maser emission is saturated and therefore independent of the radio core luminosity, b) the masers are unsaturated and originate in a thin circumnuclear gas disk, so the 'seed' radio continuum would come from the far jet which is relativistically dimmed or c) the amount, kinematics, or the distribution of the molecular gas in the nuclei of Seyferts and radio galaxies is different. Further studies of maser properties may provide clues to the differences between radio-loud and radio-quiet AGN.