Dense Gas at High Redshifts: A Search with the 110m Robert C. Byrd Telescope

We have conducted a sensitive search for the redshifted HCN(1−0) line (v_(rest) = 88.6304 GHz) towards three luminous submillimeter galaxies at z ∼ 2.5 − 2.8 using the K-band receiver (17.6-27.1GHz) on the 110m Robert C. Byrd Telescope (also known as the Green Bank Telescope). Aided by gravitational lensing, we place strong upper limits on the intrinsic line luminosity of all three sources (L′_(HCN(1−0)) ≤ 2×10^(10) K kms^(−1) pc^2). We conclude that all four submillimeter galaxies that have been observed in HCN to date are consistent with the tight relation between FIR and HCN luminosity observed in a variety of local star-forming systems, ranging from Galactic dense cores to (ultra)luminous infrared galaxies at z ≤ 0.1.