2007ApJ...660L..93G

We present here the sensitive HCN (1-0) observations made with the VLA of two submillimeter galaxies and two QSOs at high redshift. HCN emission is the signature of dense molecular gas found in giant molecular cloud (GMC) cores, the actual sites of massive star formation. We have made the first detection of HCN in a submillimeter galaxy, SMM J16359+6612. The HCN emission is seen with a signal-to-noise ratio of 4 σ and appears to be resolved as a double source of ≲2" separation. Our new HCN observations, combined with previous HCN detections and upper limits, show that the FIR/HCN ratios in these high-redshift sources lie systematically above the FIR/HCN correlation established for nearby galaxies by about a factor of 2. Even considering the scatter in the data and the presence of upper limits, this is an indication that the FIR/HCN ratios for the early universe molecular emissionline galaxies (EMGs) deviate from the correlation that fits Galactic GMC cores, normal spirals, and luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs, respectively). This indicates that the star formation rate per solar mass of dense molecular gas is higher in the high-z objects than in local galaxies including normal spirals, LIRGs, and ULIRGs. The limited HCN detections at high redshift show that the HCN/CO ratios for the high-z objects are high and are comparable to those of the local ULIRGs rather than those of normal spirals. This indicates that EMGs have a high fraction of dense molecular gas compared to total molecular gas traced by CO emission.