Widespread water vapour absorption in SgrB2(1)

We report the discovery of widespread water vapour absorption in the 2(12)-1(01) 179.5 mu m transition toward the SgrB2 molecular cloud. The data were obtained with the Long Wavelength Spectrometer (LWS) on board the ISO satellite. A raster map centered on SgrB2 made with the LWS grating clearly shows the 179.5 mu m H2O line in absorption against the far-infrared continuum at all observed positions with a line absorption depth of similar to 15%. The grating data are complemented with Fabry-Perot observations of the H2O 179.5 and 174.6 mu m lines, and the (H2O)-O-18 181.05 mu m. The transitions connecting the ground level show a broad absorption profile between -150 and 100 kms(-1) proving that water vapour is present in the molecular gas along the line of sight toward SgrB2. The H2O absorption lines probably arise in the tenuous and extended envelope of SgrB2 where collisional excitation is negligible and the excitation is mainly due to absorption of photons emitted by the dust. We derive a lower limit to the H2O abundance in SgrB2 of 10(-5).