Abstract
Context. A relationship between the water maser detection rate and far infrared (FIR) flux densities was established as a result of two 22 GHz maser surveys in a complete sample of galaxies (Dec > -30 degrees) with 100 mu m flux densities of > 50 Jy and > 30 Jy.
Aims. We attempted to discover new maser sources and investigate the galaxies hosting the maser spots by extending previous surveys to southern galaxies with particular emphasis on the study of their nuclear regions.
Methods. A sample of 12 galaxies with Dec < -30 degrees and S-100 mu m > 50 Jy was observed with the 70-m telescope of the Canberra deep space communication complex (CDSCC) at Tidbinbilla (Australia) in a search for water maser emission. The average 3 sigma noise level of the survey was 15 mJy for a 0.42 km s(-1) channel, corresponding to a detection threshold of similar to 0.1 L-circle dot for the isotropic maser luminosity at a distance of 25 Mpc.
Results. Two new detections are reported: a kilomaser with an isotropic luminosity L-H2O similar to 5 L-circle dot in NGC 3620 and a maser with about twice this luminosity in the merger system NGC 3256. The detections have been followed-up by continuum and spectral line interferometric observations with the Australia Telescope Compact Array (ATCA). In NGC 3256, a fraction (about a third) of the maser emission originates in two hot spots associated with star formation activity, which are offset from the galactic nuclei of the system. The remaining emission may originate in weaker centres of maser activity distributed over the central 50 ''. For NGC 3620, the water maser is coincident with the nuclear region of the galaxy. Our continuum observations indicate that the nature of the nuclear emission is probably linked to particularly intense star formation. Including the historical detection in NGC 4945, the water maser detection rate in the southern sample is 15% (3/20), consistent with the northern sample. The high rate of maser detections in the complete all-sky FIR sample (23%, 15/65) confirms the existence of a link between overall FIR flux density and maser phenomena. A relation between H2O and OH masers in the FIR sample is also discussed.