paper

HCN Survey of Normal Spiral, IR-luminous and Ultraluminous Galaxies

arXiv:astro-ph/0310341 · doi:10.1086/383003

Abstract

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 and the HCN luminosity initially 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 and the 12m luminosity; in fact, of all the four \IRAS bands, the 12m 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 for galaxies with excess 12m 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 ($\approxgt 3\times 10^4/τ$ cm) and give an estimate of the mass of dense molecular gas from HCN observations. Therefore, may be used as a measure of the total mass of dense molecular gas, and the luminosity ratio may indicate the fraction of molecular gas that is dense.

ApJS, May issue (final version, 12 pages + 5 figures + 4 tables). Fig. 2a,b,c and Fig. 3a,b are in GIF format due to the space limitation of astro-ph. Added an error-bar in both Fig. 4 and Fig. 5a. Please see the companion paper by Gao & Solomon (Paper II, astro-ph/0310339) for the detailed analysis and implications of this HCN survey

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