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

1. Introduction

Water is thought to be one of the most abundant molecule of the interstellar medium. Unfortunately, the broad atmospheric lines prevent any direct detection in our own Galaxy. Attempts have been made, through the isotopic molecules HDO and H O (Henkel et al 1987, Jacq et al 1988, 1990, Wannier et al 1991, Gensheimer et al 1996), and through the precursor ion H₃ O (Phillips et al 1992), and abundances of the main isotope of H₂ O around 10^-5 have been deduced. This is also confirmed by the detection in Orion of absorption lines at 2.66 microns with the K.A.O. (Knacke & Larson 1991). Very recently, observations with the ISO satellite of the - 179.5 µm line of ortho water in absorption against the continuum of the galactic center (SgrB2, Cernicharo et al 1997) has revealed that the H₂ O molecule is abundant over very extended regions.

Water levels can be collisionally pumped in the dense (n(H₂) = 10⁸ -10¹⁰ cm^-3) star-forming regions to emit strong maser lines: the 6₁₆ -5₂₃ line at 22GHz has been widely observed, because of the atmospheric transparency at this wavelength, and the 3₁₃ -2₂₀ line at 183 GHz has been detected towards the Orion molecular cloud with the K.A.O. (Waters et al 1980), as well as the 4₁₄ -3₂₁ line at 380GHz (Phillips et al 1980). All these lines are well known to be masing, since they correspond to "backbones", i.e. levels with the lowest energy for a given rotational quantum number (de Jong 1973, Cooke & Elitzur 1985, Deguchi & Nguyen-Q-Rieu 1990). Neufeld and Melnick (1987) computed the total flux emitted by the H₂ O lines in the shocked gas region of Orion. They concluded that H₂ O is by far the main coolant, 40 % of the total cooling being provided by the far-infrared ortho-H₂ O lines, in the shocked gas regions. Since then a series of higher-level maser lines have been detected (Menten et al 1990a, b; Melnick et al 1993). Neufeld and Melnick (1990, 1991) interprete the maser data in the frame of excitation models, and show that the minimum masing gas temperature is 200K. This favors slow ( 50 km/s) non-dissociative shocks for the source of maser emission (Melnick et al 1993, Kaufman & Neufeld 1996a, b).

Even from the ground at 183 GHz, Cernicharo et al (1990) succeeded, with the IRAM 30m telescope, to detect in Orion the para H₂ O line which turns out to be inverted with a huge flux of a few 10⁴ Jy (main beam temperatures of the order of 2000K). Contrary to the point-like maser sources at 22 GHz, the 183 GHz maser emission is quite extended ( 1 arcmin = 0.2pc), and represents a significant coolant in the star-forming region (Cernicharo et al 1994). González-Alfonso et al (1995) also found that the 183 GHz emission is spatially extended in the star-forming cloud W49N. The maser dominated 183 GHz H₂ O emission is not confined to narrow shock regions, but on the contrary extends out to 2.2pc from the cloud center, and is very similar in extension and kinematics to molecular emission from dense gas tracers (CS, HCN, H₂ CO..). Also extended 183 GHz H₂ O maser emission has been detected in low-mass star forming regions, such as HH7-11 (Cernicharo et al 1996). Cernicharo et al (1994) show, with the help of an escape probability model for radiative transfer, that the 183 GHz line can be inverted for densities n(H₂) - 10⁶ cm^-3 and kinetic temperatures between 50 and 100K. The maser begins to saturate for a para-H₂ O column density larger than 10¹⁸ cm^-2. Modelisation of the Orion region implies that the H₂ O/CO abundance ratio is 0.3-0.5.

The detection of the 183 GHz transition of water should provide additional constraints to investigate the physical conditions in the extended star-forming regions that are found in the central region of starburst galaxies. For this purpose, we have tried to detect the masing 183 GHz line in two infrared ultraluminous starburst galaxies, redshifted in a transparent window of the atmosphere, near =2mm. We have chosen two mergers clearly detected in CO at a velocity near 50 000 km/s with a redshift ideal for our purposes: Mkn 1014 (Sanders et al 1988b, Alloin et al 1992) and VIIZw244 (Alloin et al 1992). No H₂ O line was detected in both galaxies with a 3 upper limit of 3 mK. We discuss the main implications of these negative results on the determination of the physical parameters of the interstellar medium in starburst galaxies.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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