HCN Observations of Dense Star-forming Gas in High-Redshift Galaxies

We present here the sensitive HCN(1-0) observations made with the VLA of twosubmillimeter galaxies and two QSOs at high-redshift. HCN emission is thesignature of dense molecular gas found in GMC cores, the actual sites ofmassive star formation. We have made the first detection of HCN in asubmillimeter galaxy, SMM J16359+6612. The HCN emission is seen with a signalto noise ratio of 4$\sigma$ and appears to be resolved as a double-source of$\approxlt 2''$ separation. Our new HCN observations, combined with previousHCN detections and upper limits, show that the FIR/HCN ratios in these highredshift sources lie systematically above the FIR/HCN correlation establishedfor nearby galaxies by about a factor of 2. Even considering the scatter in thedata and the presence of upper limits, this is an indication that the FIR/HCNratios for the early Universe molecular emission line galaxies (EMGs) deviatefrom the correlation that fits Galactic giant molecular cloud cores, normalspirals, LIRGs, and ULIRGs. This indicates that the star formation rate persolar mass of dense molecular gas is higher in the high-$z$ objects than inlocal galaxies including normal spirals LIRGs and ULIRGs. The limited HCNdetections 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 thanthose of normal spirals. This indicates that EMGs have a high fraction of densemolecular gas compared to total molecular gas traced by CO emission.Comment: 14 pages including 4 figures; ApJL accepte