Upper limits to the water abundance in starburst galaxies

Abstract

Aims. We have searched for emission from the 557 GHz ortho-water line in the interstellar medium of six nearby starburst galaxies. Methods. We used the Odin satellite to observe the 110-101 transition of o-H2O in the galaxies NGC 253, IC 342, M 82, NGC 4258, CenA, and M 51. None of the galaxies in our sample was detected. Results. We derive three sigma upper limits to the H2O abundance relative to H2 ranging from 2 × 10-9 to 1 × 10-8. The best of these upper limits are comparable to the measured abundance of H2O in the Galactic star forming region W3. However, if only 10% of the molecular gas is in very dense cores, then the water abundance limits in the cores themselves would be larger by a factor of 10 i.e. 2 × 10-8 to 1 × 10-7. Conclusions. These observations suggest that detections of H2O emission in galaxies with the upcoming Herschel Space Observatory are likely to require on-source integration times of an hour or more except in the very brightest extragalactic targets such as M 82 and NGC 253. © ESO 2007.