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

We present a search for 183 GHz H2O (313 → 220) 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), 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 physical properties similar to those of, e.g., the Orion or W49N molecular cloud cores, the flux ratio between the maser-amplified H2O (313 → 220) and the thermally excited 12CO (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 12CO (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 < 0.6 × 109 K km s-1 pc2. Assuming a brightness temperature of Tb(H2O) ≃ 500 K for the maser emission and CO properties from the literature, this translates to a H2O (313 → 220)/12CO (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.