Astron. Astrophys. 323, 357-362 (1997)

4. Conclusion

We have not detected the H₂ O 183 GHz masing line in two ultraluminous starburst galaxies, and the 3 upper limits are an order of magnitude lower than the expected signals. Predictions were based on the high surface filling factor of warm and dense molecular gas, similar to the Orion star-forming core, where 183 GHz emission has been detected with a very high brightness temperature on very extended scales (Cernicharo et al 1994). That most of the gas of ultraluminous starburst should be in warm and dense cores is deduced from the observations in these objects of high H₂ column densities, large volumic densities, and high dust temperatures, averaged over one kpc scale. This result could be explained if the water column density are so high as to quench the maser excitation.

Observations of water in distant starburst galaxies with ISO do not seem promising. For example, the 2₁₂ - 1₀₁ line at 179.5 µm in Mkn 1014 can be estimated to be 3 10^-15 erg s^-1 cm^-2. This predicted flux is 3 magnitudes below the detection limit of the Long Wavelength Spectrometer of ISO. The 179 µm line could be detected in absorption however (cf Cernicharo et al 1997).

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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