2011A&A...532A.125T

Narrow-line Seyfert 1 (NLS1) galaxies are a class of active galactic nuclei (AGN) that have all the properties of type 1 Seyfert galaxies but show peculiar characteristics, including the narrowest Balmer lines, strongest Fe II emission, and extreme properties in the X-rays. Line and continuum radio observations provide an optimal tool to access the (often) optically obscured innermost regions of AGN and reveal the kinematics of the gas around their central engines. We investigate the interplay between the peculiar NLS1 class of AGN and the maser phenomenon, to help us understand the nature of the maser emission in some NLS1s where water maser emission has been detected. We observed a sample of NLS1 galaxies with the Green Bank Telescope in a search for water maser emission at 22GHz. We also reduced and analysed archival Green Bank Telescope and Very Large Array data and produced 22-GHz spectra for the five NLS1 galaxies with detected maser emission. In particular, we imaged the maser and nuclear radio continuum of NGC 5506 at subarcsec scales with the Very Large Array. We discovered maser emission in two NLS1 galaxies: IGR J16385-2057, and IRAS 03450+0055. In addition to the three previously known maser detections in the NLS1s Mrk 766, NGC 4051, and NGC 5506, this yields a water maser detection rate in NLS1 galaxies of ∼7% (5/71). This value rises significantly to ∼21% (5/24) when considering only NLS1 galaxies at recessional velocities less than 10000 km/s. For NGC 4051 and NGC 5506, we find that the water maser emission is located within 5 and 12pc, respectively, of nuclear radio continuum knots, which are interpreted as core-jet structures. The water maser detection rate in NLS1s is surprisingly high, much higher than the detection rate obtained for type 1 AGN and similar to those in Seyfert 2 and low-ionization nuclear emission-line region galaxies. The masers in NGC 4051 and NGC 5506 are nuclear and associated with the AGN, either with an accretion disk, a radio jet, or a nuclear outflow. The apparent lack of high-velocity maser features and evidence, recently reported, of radiative outflows and radio jets in the host galaxies seems to favour interpretation as a jet or an outflow. A similar association is also seemingly true for the maser in Mrk 766, IGR J16385-2057, and IRAS 03450+0055, although, in these cases, without radio interferometric measurements we cannot rule out an off-nuclear origin of the emission.