Detection of Emission from the Cn Radical in the Cloverleaf Quasar at Z=2.56

We report the detection of CN(N=3→2) emission towards 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) > 10 4 cm) 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) × 10 K kmspc. 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. Subject headings: galaxies: active, starburst, formation, high redshift — cosmology: observations — radio lines: galaxies