Excitation mechanisms for HCN(1-0) and HCO+(1-0) in galaxies from the great observatories all-sky LIRG survey

We present new Institut de Radioastronomie Millimetrique (IRAM) 30 m spectroscopic observations of the ~88 GHz band, including emission from the CCH (N = 1 → 0) multiplet, HCN (J = 1 → 0), HCO+ (J = 1 → 0), and HNC (J = 1 → 0), for a sample of 58 local luminous and ultraluminous infrared galaxies from the Great Observatories All-sky LIRG Survey (GOALS). By combining our new IRAM data with literature data and Spitzer/IRS spectroscopy, we study the correspondence between these putative tracers of dense gas and the relative contribution of active galactic nuclei (AGNs) and star formation to the mid-infrared luminosity of each system. We find the HCN (1-0) emission to be enhanced in AGN-dominated systems (〈LHCN′(1-0)/L′HCO+(1-0)〉 = 1.84), compared to composite and starburst-dominated systems (〈L′HCN(1-0)/L′HCO(1-0)〉 = 1.14, and 0.88, respectively). However, some composite and starburst systems have L′HCN(1-0)/L′HCO (1-0) + ratios comparable to those of AGNs, indicating that enhanced HCN emission is not uniquely associated with energetically dominant AGNs. After removing AGN-dominated systems from the sample, we find a linear relationship (within the uncertainties) between log10(L′HCN(1-0)) and log10(LIR), consistent with most previous findings. L′HCN 1 0 ( - )/LIR, typically interpreted as the dense-gas depletion time, appears to have no systematic trend with LIR for our sample of luminous and ultraluminous infrared galaxies, and has significant scatter. The galaxyintegrated L′HCN(1-0) and L′HCO (1-0) + emission do not appear to have a simple interpretation in terms of the AGN dominance or the star formation rate, and are likely determined by multiple processes, including density and radiative effects. © 2015. The American Astronomical Society. All rights reserved.