Upper limits to the water abundance in starburst galaxies

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 l 10- l 01 transition of o-H 2 O 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 H 2 O abundance relative to H 2 ranging from 2 x 10 -9 to l x 10 -8 . The best of these upper limits are comparable to the measured abundance of H 2 O 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 x 10 -8 to l x 10 -7 . Conclusions. These observations suggest that detections of H 2 O 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.