Detection of Water at [CLC][ITAL]z[/ITAL][/CLC] = 0.685 toward B0218+357

We report the detection of the H 2 O molecule in absorption at a redshift, z , of 0.68466 in front of the gravitationally lensed quasar B0218+357. We detect the fundamental transition of ortho-water at 556.93 GHz (redshifted to 330.59 GHz). The line is highly optically thick and relatively wide (15 km s -1 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 330.59 GHz, which corresponds to the level expected from the power-law spectrum S (ν) ∝ ν -0.25 already observed at lower frequencies, we deduce that the filling factor of the H 2 O absorption is large. It was already known from the high optical thickness of the CO, 13 CO, and C 18 O lines that molecular clouds entirely cover one of the two lensed images of the quasar (all its continuum is absorbed); our present results indicate that H 2 O clouds are covering a comparable surface. The H 2 O molecules are therefore not confined to small cores with a tiny filling factor but are extended over parsec scales. The H 2 O 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 yields constraints on the excitation temperature.