Upper limits to the water abundance in starburst galaxies^*

¹ Department of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada e-mail: wilson@physics.mcmaster.ca
² Onsala Space Observatory, 439 92 Onsala, Sweden
³ Hartebeesthoek Radio Astromomy Observatory, Box 443, Krugersdorp, 1740, South Africa
⁴ Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden
⁵ Laboratoire d'Astronomie Spatiale, BP 8, 13376 Marseille Cedex 12, France
⁶ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁷ Department of Physics, University of Waterloo, Waterloo, ON N2L 3G1, Canada
⁸ Swedish Space Corporation, PO Box 4207, 171 04 Solna, Sweden
⁹ LERMA & UMR 8112 du CNRS, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
¹⁰ ESA Space Telescope Division, STScI, 3700 San Martin Drive Baltimore, MD 21218, USA

Received: 23 October 2006
Accepted: 5 April 2007

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  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 210^-9 to 110^-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. 210^-8 to 110^-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.