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2008A&A...479..111C - Astronomy and Astrophysics, volume 479, 111-122 (2008/2-3)

New H2O masers in Seyfert and FIR bright galaxies II. The intermediate luminosity range.

CASTANGIA P., TARCHI A., HENKEL C. and MENTEN K.M.

Abstract (from CDS):

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 100µm flux density >50Jy and a declination >-30°. 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. A sample of 41 galaxies with 30Jy<S100µm<50Jy and δ>-30° was observed with the 100-m telescope at Effelsberg in a search for the 22GHz water vapor line. The average 3σ noise level of the survey is 40mJy for a 1km/s channel, corresponding to a detection threshold for the isotropic maser luminosity of ∼0.5L at a distance of 25 Mpc. Two detections are reported: a megamaser with an isotropic luminosity, L(H2O), of ≃35L in the Seyfert/HII galaxy NGC613 and a kilomaser with L(H2O)≃1L☉_ 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 ≲0.02pc sized region with a brightness temperature >1010K (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 NGC520. 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 S100µm>50Jy 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.

Journal keyword(s): masers - galaxies: active - galaxies: nuclei - galaxies: starburst - galaxies: statistics - radio lines: galaxies

Nomenclature: Tables 4, 5: [CTH2008] NGC 520 A (Nos A-G).