New H$_{\mathsf 2}$O masers in Seyfert and FIR bright galaxies

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• ) with 100 μm 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• and S 100 μm > 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 3σ noise level of the survey was 15 mJy for a 0.42 km s −1 channel, corresponding to a detection threshold of ∼0.1 L for the isotropic maser luminosity at a distance of 25 Mpc. Results. Two new detections are reported: a kilomaser with an isotropic luminosity L H 2 O ∼ 5 L 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 H 2 O and OH masers in the FIR sample is also discussed.

New H2O masers in Seyfert and FIR bright galaxies

“…Despite our warning on using a strict isotropic luminosity threshold to infer the origin of water maser emission (see previous section), in the following for consistency with past works (see e.g. Surcis et al 2009, and references therein), we necessarily have to use 10 and 1 L as isotropic luminosities to distinguish between the kilomaser and megamaser regime for H 2 O and OH, respectively. According to this classification, 95% (89/94) are OH megamasers in all the galaxies where OH maser emission has been found.…”

“…For the 8 sources labeled with "SF(? )", the association with star formation is instead (only) the most likely option since it is based on qualitative considerations on the nuclear position of the maser emission, the absence of clear evidence of AGN activity in the host galaxy and/or the shape of the maser line profile (see e.g., Castangia et al (2008), for NGC 520, Surcis et al (2009), for NGC 3620).…”

“…5. The present work is the last in a series of papers (Henkel et al 2005;Castangia et al 2008;Surcis et al 2009) that report searches for water maser emission in complete FIR-flux based samples of galaxies, highlight new water maser line detections, and present detailed maps of their surroundings.…”

“…Most of the former are related to star formation similar to, although typically brighter than, the galactic masers, while the latter are thought to be exclusively powered by the nuclear activity of active galactic nuclei (AGN). The kilomaser sources associated with the active galactic nuclei of M 51 (Hagiwara et al 2001b) and of a few more H 2 O masers of lower luminosity in the inner parsecs of NGC 4051 (Hagiwara et al 2003), NGC 520 (Castangia et al 2008), and NGC 3620 (Surcis et al 2009) hint at the possibility that some kilomasers could also be related to nuclear activity, providing the low-luminosity tail of the more powerful megamasers.…”

Masers in Starburst Galaxies

“…masers (e.g., Castangia et al 2008, Darling et al 2008, Surcis et al 2009, Tarchi et al 2011. But in most cases the physical conditions, such as those found in dwarf starbursts or the Antennae Galaxies (a major merger), are not analogous to those found in the Galaxy (e.g., Brogan et al 2010).…”