DETECTION OF WATER AT Z = 0.685 TOWARD B0218+357

We report the detection of the H2O molecule in absorption at a redshif t, z, of 0.68466 in front of the gravitationally lensed quasar B0218+3 57. We detect the fundamental transition of ortho-water at 556.93 GHz (redshifted to 330.59 GHz). The line is highly optically thick and rel atively wide (15 km s(-1) FWHM), with a profile that is similar to tha t of the previously detected CO(2-1) and HCO+(2-1) optically thick abs orption lines toward this quasar. From the measured level of the conti nuum at 330.59 GHz, which corresponds to the level expected from the p ower-law spectrum S(nu) proportional to nu(-0.25) already observed at lower frequencies, we deduce that the filling factor of the H2O absorp tion is large. It was already known from the high optical thickness of the CO, (CO)-C-13, and (CO)-O-18 lines that molecular clouds entirety cover one of the two lensed images of the quasar (all its continuum i s absorbed); our present results indicate that 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 s cales. 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 183 GHz line in absorption, obtaining only an upper limit; this yield s constraints on the excitation temperature.