2007ApJ...666..778R

We report the detection of CN(N=3⟶2) emission toward the Cloverleaf quasar (z=2.56) based on observations with the IRAM Plateau de Bure Interferometer. This is the first clear detection of emission from this radical at high redshift. CN emission is a tracer of dense molecular hydrogen gas [n(H₂)>10⁴/cm3] within star-forming molecular clouds, in particular, in regions where the clouds are affected by UV radiation. The HCN/CN intensity ratio can be used as a diagnostic for the relative importance of photodissociation regions (PDRs) in a source and as a sensitive probe of optical depth, the radiation field, and photochemical processes. We derive a lensing-corrected CN(N=3⟶2) line luminosity of L'_CN(3–2)=(4.5±0.5)x10⁹ K.km/s/pc2. The ratio between CN luminosity and far-infrared luminosity falls within the scatter of the same relationship found for low-z (ultra-) luminous infrared galaxies. Combining our new results with CO(J=3⟶2) and HCN(J=1⟶0) measurements from the literature and assuming thermal excitation for all transitions, we find a CO/CN luminosity ratio of 9.3±1.9 and a HCN/CN luminosity ratio of 0.95±0.15. However, we find that the CN(N=3⟶2) line is likely only subthermally excited, implying that those ratios may only provide upper limits for the intrinsic 1⟶0 line luminosity ratios. We conclude that, in combination with other molecular gas tracers like CO, HCN, and HCO⁺, CN is an important probe of the physical conditions and chemical composition of dense molecular environments at high redshift.