2006ApJ...649..635R

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'_H2_O<0.6x10⁹ K km/s pc². Assuming a brightness temperature of T_b(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.