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

We present a search for 183 GHz H 2 O (3 13 → 2 20 ) emission in the infrared-luminous quasar MG 0751+2716 with the NRAO Very Large Array (VLA). At z = 3.200 ± 0.001, this water emission feature is redshifted to 43.6 GHz. Unlike the faint rotational transitions of HCN (the standard high-density tracer at high z ), H 2 O (3 13 → 2 20 ) 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 physical properties similar to those of, e.g., the Orion or W49N molecular cloud cores, the flux ratio between the maser-amplified H 2 O (3 13 → 2 20 ) and the thermally excited 12 CO ( J = 1 → 0) transitions may be as high as factor of 20 (but has to be corrected by their relative filling factor). MG 0751+2716 is a strong 12 CO ( J = 4 → 3) emitter, and therefore one of the most suitable targets to search for H 2 O (3 13 → 2 20 ) at cosmological redshifts. Our search resulted in an upper limit in line luminosity of L < 0.6 × 10 9 K km s -1 pc 2 . Assuming a brightness temperature of T b (H 2 O) ≃ 500 K for the maser emission and CO properties from the literature, this translates to a H 2 O (3 13 → 2 20 )/ 12 CO ( J = 4 → 3) area filling factor of less than 1%. However, this limit is not valid if the H 2 O (3 13 → 2 20 ) 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, H 2 O does not appear to be a more luminous alternative to HCN to detect high-density gas in star-forming environments at high redshift.