Abstract
Context. Recently, a relationship between the water maser detection rate and far infrared (FIR) flux density has been found as a result of a 22GHz maser survey in a sample comprised of northern galaxies with 100 mu m flux density > 50 Jy and a declination >-30 degrees.
Aims. The survey has been extended toward galaxies with lower FIR flux densities in order to confirm this correlation and to discover additional maser sources for relevant follow-up interferometric studies.
Methods. A sample of 41 galaxies with 30 Jy < S (100 mu m) < 50 Jy and delta > -30. was observed with the 100-m telescope at Effelsberg in a search for the 22 GHz water vapor line. The average 3s noise level of the survey is 40 mJy for a 1 km s(-1) channel, corresponding to a detection threshold for the isotropic maser luminosity of similar to 0.5 L-circle dot at a distance of 25 Mpc.
Results. Two detections are reported: a megamaser with an isotropic luminosity, L-H2O, of approximate to 35 L-circle dot in the Seyfert/Hii galaxy NGC613 and a kilomaser with L-H2O approximate to 1 L-circle dot in the merger system NGC520. The high luminosity and the presence of a Seyfert nucleus favor an association for NGC613 with an active galactic nucleus. The kilomaser in NGC520 was also detected with the Very Large Array, providing a position with subarcsecond accuracy. The H2O emission, originating from a less than or similar to 0.02 pc sized region with a brightness temperature greater than or similar to 10(10) K (if the observed variations are intrinsic to the masing cloud(s)), is close to one of the two radio continuum sources located in the inner parsecs of NGC 520. The maser is most likely associated with a young supernova remnant (SNR), although an association with a low-luminosity AGN (LLAGN) cannot be ruled out. The maser detection rate, with 2 new maser sources out of 41 galaxies observed, is consistent with expectations extrapolated from the statistical properties of the S (100 mu m) > 50 Jy sample. The H2O kilomasers are "subluminous", while H2O megamasers tend to be "superluminous" with respect to the FIR luminosity of their parent galaxy, when compared with sites of massive star formation in the Milky Way.