Upper limits to the water abundance in starburst galaxies*
Department of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada e-mail: email@example.com
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
Accepted: 5 April 2007
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.
Key words: galaxies: starburst / galaxies: general / ISM: molecules / astrochemistry
Based on observations with Odin, a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and Centre National d'Etude Spatiale (CNES). The Swedish Space Corporation has been the industrial prime contractor and also is operating the satellite.
© ESO, 2007