Search for 183 GHz water maser emission in starburst galaxies

We have searched for water vapor emission at 183 GHz, redshifted at 157 GHz and 161 GHz, in the two ultraluminous starburst galaxies Mkn 1014 and VIIZw244. Due to the low energy level of the upper state of the 183 GHz transition (≈ 200K), it is expected that the emission regions are extended, as they are in the Orion or W49Nmolecular cloud cores. Since the warm and dense gas, typical of star-forming cores, is expected to have a large surface filling factor in ultraluminous starburst galaxies, the maser H2O emission at 183 GHz could have been detected. In fact, no water line has been detected in Mkn 1014 and VIIZw244 with an upper limit of σ = 1 mK. We compare the H2O/CO emission ratio with that obtained towards the ultraluminous high-z object F10214+4724, and suggest that the amplification factor for the H2O emitting cores in this galaxy should be higher than for the general CO emitting region. We conclude that the warm and dense H2O cores are much less extended in the two observed starburst galaxies than in the Orion molecular cloud; this provides some information on the physical conditions and cooling processes of the interstellar medium in starburst galaxies.