Detection of emission from the CN radical in the Cloverleaf quasar at z=2.56

作者：Riechers Dominik A^*; Walter Fabian; Cox Pierre; Carilli Christopher L; Weiss Axel; Bertoldi Frank; Neri Roberto

来源：Astrophysical Journal, 2007, 666(2): 778-783.

DOI：10.1086/520335

• cosmology : observations
• galaxies : active
• galaxies : formation
• galaxies : high-redshift galaxies : starburst
• radio lines : galaxies

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(H-2) > 10(4) cm(-3)] 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(9) K km s(-1) pc(2). The ratio between CN luminosity and farinfrared 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.

• 出版日期2007-9-10