2004ApJS..152...63G

We report systematic HCN J=1-0 (and CO) observations of a sample of 53 infrared (IR) and/or CO-bright and/or luminous galaxies, including seven ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more than a dozen of the 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 L_IRand the HCN luminosity L_HCNinitially proposed by Solomon, Downes, & Radford, which is detailed in a companion paper. We also address here the issue of HCN excitation. There is no particularly strong correlation between L_HCNand the 12 µm luminosity; in fact, of all the four IRAS bands, the 12 µm luminosity has the weakest correlation with the HCN luminosity. There is also no evidence of stronger HCN emission or a higher ratio of HCN and CO luminosities L_HCN/L_CO for galaxies with excess 12 µm emission. This result implies that mid-IR radiative pumping, or populating, of the J=1 level of HCN by a mid-IR vibrational transition is not important compared with the collisional excitation by dense molecular hydrogen. Furthermore, large velocity gradient calculations justify the use of HCN J=1-0 emission as a tracer of high-density molecular gas (≳3x10⁴/τ/cm3) and give an estimate of the mass of dense molecular gas from HCN observations. Therefore, L_HCNmay be used as a measure of the total mass of dense molecular gas, and the luminosity ratio L_HCN/L_COmay indicate the fraction of molecular gas that is dense.