ar X iv : a st ro - p h / 03 10 34 1 v 1 1 4 O ct 2 00 3 HCN Survey of Normal Spiral , IR - luminous and Ultraluminous Galaxies

We report systematic HCN(1-0) observations (and CO observations) of a sample of 53 infrared (IR) and/or CO bright and/or luminous galaxies, including 7 ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more than a dozen nearest normal spiral galaxies. This is the largest and most sensitive HCN survey of galaxies to date. All galaxies observed so far follow the tight correlation between the IR luminosity LIR and the HCN luminosity LHCN initially proposed by Solomon, Downes, & Radford (1992), which is detailed in a companion paper (Gao & Solomon 2003, Paper II). We also address here the issue of the HCN excitations. There is no particularly strong correlation between LHCN and the 12μm luminosity, in fact, the weakest among 4 IRAS bands. There is also no evidence of stronger HCN emission or a higher ratio of HCN and CO luminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that the mid-IR radiative pumping, or populating, of the J=1 level of HCN by a mid-IR vibrational transition to excite HCN emission is not important when compared with the collisional excitation by high density molecular hydrogen. Furthermore, large velocity gradient calculations justify the use of HCN(1-0) emission as a tracer of high density molecular gas ( ∼3× 10 4 cm) and give an estimate of the mass of dense molecular gas from HCN observations. Therefore, LHCN may be used as a measure of the total mass of dense molecular gas, and the luminosity ratio LHCN/LCO may indicate the fraction of molecular gas that is dense.