We present a search for 183 GHz H _ { 2 } O ( 3 _ { 13 } \to 2 _ { 20 } ) emission in the infrared–luminous quasar MG 0751+2716 with the NRAO Very Large Array ( VLA ) .
At z = 3.200 \pm 0.001 , this water emission feature is redshifted to 43.6 GHz .
As opposed to the faint rotational transitions of HCN ( the standard high–density tracer at high– z ) , H _ { 2 } O ( 3 _ { 13 } \to 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 similar physical properties as e.g .
the Orion or W49N molecular cloud cores , the flux ratio between the maser–amplified H _ { 2 } O ( 3 _ { 13 } \to 2 _ { 20 } ) and the thermally excited ^ { 12 } CO ( J =1 \to 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 \to 3 ) emitter , and therefore one of the most suitable targets to search for H _ { 2 } O ( 3 _ { 13 } \to 2 _ { 20 } ) at cosmological redshifts .
Our search resulted in an upper limit in line luminosity of L ^ { \prime } _ { H _ { 2 } O } < 0.6 \times 10 ^ { 9 } K km s ^ { -1 } pc ^ { 2 } .
Assuming a brightness temperature of T _ { b } { ( H _ { 2 } O ) } \simeq 500 K for the maser emission and CO properties from the literature , this translates to a H _ { 2 } O ( 3 _ { 13 } \to 2 _ { 20 } ) / ^ { 12 } CO ( J =4 \to 3 ) area filling factor of less than 1 % .
However , this limit is not valid if the H _ { 2 } O ( 3 _ { 13 } \to 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 .