A search for extragalactic H₂O maser emission towards IRAS galaxies

Detection of a maser from an infrared-luminous merger, NGC 6240

¹ Max-Planck-Institut für Radioastronomie (MPIfR) Auf dem Hügel 69, 53121 Bonn, Germany
² Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL, UK
³ National Astronomy Observatory, Osawa, Mitaka, Japan

Corresponding author: Y. Hagiwara, hagi@mpifr-bonn.mpg.de

Received: 31 July 2001
Accepted: 26 November 2001

Abstract

We report the result of an on-going survey for 22 GHz H₂O maser emission towards infrared luminous galaxies. The observed galaxies were selected primarily from the IRAS bright galaxy sample. The survey has resulted in the detection of one new maser. The new maser was discovered towards the [U]LIRG/merger galaxy NGC 6240, which contains a LINER nucleus. This is the first detection of an H₂Omaser towards this class of galaxy, they are traditionally associated with OH megamaser sources. The detected maser emission is highly redshifted (~260–300 km s^-1) with respect to the adopted systemic velocity of the galaxy, and we identified no other significant emission at velocities ±500 km s relative to the systemic velocity. The presence of high-velocity maser emission implies the possible existence of a rotating maser disk formed in the merging process. The large maser luminosity (~40 ) suggests that an active galactic nucleus could be the energy source that gives rise to the water emission. Alternatively, the maser emission could be associated with the previously observed double radio source in the centre of the galaxy. Interferometric observations with high angular resolution will be able to clarify the origin of the new maser.