ar X iv : a st ro - p h / 97 06 27 4 v 1 2 7 Ju n 19 97 Detection of water at z = 0 . 685 towards B 0218 + 357

We report the detection of the H2O molecule in absorption at a redshift z=0.68466 in front of the gravitationally lensed quasar B0218+357. We detect the fundamental transition of ortho–water at 556.93GHz (redshifted to 330.59GHz). The line is highly optically thick and relatively wide (15 km/s FWHM), with a profile that is similar to that of the previously detected CO(2–1) and HCO(2–1) optically thick absorption lines toward this quasar. From the measured level of the continuum at 330.59GHz, which corresponds to the level expected from the power–law spectrum S(ν) ∝ ν already observed at lower frequencies, we deduce that the filling factor of the H2O absorption is large. It was already known from the high optical thickness of the CO, CO and CO lines that the molecular clouds entirely cover one of the two lensed images of the quasar (all its continuum is absorbed); our present results indicate that the H2O clouds are covering a comparable surface. The H2O molecules are therefore not confined to small cores with a tiny filling factor, but are extended over parsec scales. The H2O line has a very large optical depth, and only isotopic lines could give us the water abundance. We have also searched for the 183GHz line in absorption, obtaining only an upper limit; this yields constraints on the excitation temperature. Subject headings: BL Lacertae objects: individual (B0218+357) — galaxies : abundances — galaxies : ISM — ISM : molecules — quasars : absorption lines — radio lines : ISM