A Search for H 2 O in the Strongly Lensed Qso Mg 0751 + 2716 at Z = 3 . 2

We present a search for 183GHz H2O(313→220) emission in the infrared–luminous quasar MG0751+2716 with the NRAO Very Large Array (VLA). At z = 3.200 ± 0.001, this water emission feature is redshifted to 43.6GHz. As opposed to the faint rotational transitions of HCN (the standard high–density tracer at high–z), H2O(313→220) is observed with high maser amplification factors in Galactic star–forming regions. It therefore holds the potential to trace high–density star– forming regions in the distant universe. If indeed all star–forming regions in massively star–forming galaxies at z > 3 have similar physical properties as e.g. the Orion or W49N molecular cloud cores, the flux ratio between the maser–amplified H2O(313→220) and the thermally excited CO(J=1→0) transitions may be as high as factor of 20 (but has to be corrected by their relative filling factor). MG0751+2716 is a strong CO(J=4→3) emitter, and therefore one of the most suitable targets to search for H2O(313→220) at cosmological redshifts. Our search resulted in an upper limit in line luminosity of L′H2O < 0.6 × 10 K kms pc. Assuming a brightness temperature of Tb(H2O) ≃ 500K for the maser emission and CO properties from the literature, this translates to a H2O(313→220)/ CO(J=4→3) area filling factor of less than 1%. However, this limit is not valid if the H2O(313→220) maser emission is quenched, i.e. if the line is only thermally excited. We conclude that, if our results were to hold for other high–z sources, H2O does not appear to be a more luminous alternative to HCN to detect high–density gas in star–forming environments at high redshift. Subject headings: galaxies: active, starburst, formation, high redshift — cosmology: observations — masers