A search for extragalactic H2O maser emission towards IRAS galaxies -: Detection of a maser from an infrared-luminous merger, NGC 6240

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 NGC6240, which contains a LINER nucleus. This is the first detection of an H2O maser towards this class of galaxy, they are traditionally associated with OH megamaser sources. The detected maser emission is highly redshifted (similar to260-300 km s(-1)) with respect to the adopted systemic velocity of the galaxy, and we identified no other significant emission at velocities less than or similar to+/-500 km s(-1) 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 (similar to40 L-circle dot) 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.