A Search for H2O in the Strongly Lensed QSO MG 0751+2716 atz= 3.2

We present a search for 183 GHz H_2O(3_13-2_20) emission in theinfrared-luminous quasar MG 0751+2716 with the NRAO Very Large Array (VLA). Atz=3.200+/-0.001, this water emission feature is redshifted to 43.6 GHz. Asopposed to the faint rotational transitions of HCN (the standard high-densitytracer at high-z), H_2O(3_13-2_20) is observed with high maser amplificationfactors in Galactic star-forming regions. It therefore holds the potential totrace high-density star-forming regions in the distant universe. If indeed allstar-forming regions in massively star-forming galaxies at z>3 have similarphysical properties as e.g. the Orion or W49N molecular cloud cores, the fluxratio between the maser-amplified H_2O(3_13-2_20) and the thermally excitedCO(1-0) transitions may be as high as factor of 20 (but has to be corrected bytheir relative filling factor). MG 0751+2716 is a strong CO(4-3) emitter, andtherefore one of the most suitable targets to search for H_2O(3_13-2_20) atcosmological redshifts. Our search resulted in an upper limit in lineluminosity of L'(H_2O) < 0.6 x 10^9 K km/s pc^2. Assuming a brightnesstemperature of T_b(H_2O) ~= 500 K for the maser emission and CO properties fromthe literature, this translates to a H_2O(3_13-2_20)/CO(4-3) area fillingfactor of less than 1%. However, this limit is not valid if the H_2O(3_13-2_20)maser emission is quenched, i.e. if the line is only thermally excited. Weconclude that, if our results were to hold for other high-z sources, H_2O doesnot appear to be a more luminous alternative to HCN to detect high-density gasin star-forming environments at high redshift.Comment: 6 pages, 1 figure, to appear in ApJ (accepted May 19, 2006