New HO masers in Seyfert and FIR bright galaxies II. The intermediate luminosity range^*

¹ Max-Planck-Insitut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: pcastang@mpifr-bonn.mpg.de
² INAF – Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
³ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy

Received: 19 July 2007
Accepted: 28 November 2007

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 22 GHz maser survey in a sample comprised of northern galaxies with 100 μm flux density >50 Jy and a declination >-30°.

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  50 Jy and ° was observed with the 100-m telescope at Effelsberg in a search for the 22 GHz water vapor line. The average 3σ 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 ~0.5  at a distance of 25 Mpc.

Results.Two detections are reported: a megamaser with an isotropic luminosity, , of ≈35  in the Seyfert/Hii galaxy NGC 613 and a kilomaser with  ≈ 1  in the merger system NGC 520. The high luminosity and the presence of a Seyfert nucleus favor an association for NGC 613 with an active galactic nucleus. The kilomaser in NGC 520 was also detected with the Very Large Array, providing a position with subarcsecond accuracy. The H₂O emission, originating from a 0.02 pc sized region with a brightness temperature 10¹⁰ 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  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.