A search for extragalactic H2O maser emission towards IRAS galaxies - II. Discovery of an H2O maser in the type 1 Seyfert galaxy NGC 4051

Abstract

Water vapour maser emission in the 616–523 transition towards the narrow-line type 1 Seyfert (NLS1) galaxy NGC 4051 has been discovered during an ongoing single-dish extragalactic water maser survey. The Doppler-shifted maser components appear to bracket maser components lying near the systemic velocity of the galaxy symmetrically. The tentative result of a Very Large Array (VLA) snapshot observation is that the masers are confined within 0.1 arcsec (5 pc at a distance D= 9.7 Mpc) of the radio continuum peak seen at 8.4 GHz. The low luminosity of the maser (∼2 L⊙) is not typical for masers that coincide with the radio continuum nucleus and appear associated with active galactic nucleus (AGN) activity. A low-luminosity maser in a type 1 Seyfert nucleus could be explained by a low maser gain resulting from the lower inclination of an obscuring disc around an active nucleus.