Molecular gas, probed by CO, has long been associated with ongoing star formation, but the total molecular gas mass is not linearly correlated with the total star formation rate in galaxies. In contrast, molecules such as HCN and HCO+, which trace higher density gas, show linear correlations with the star formation rate. This suggests HCN and HCO+ trace the dense gas intimately related to ongoing star formation. However, in active galactic nuclei, the HCN emission is enhanced (relative to star-forming galaxies), suggesting the HCN excitation may not trace star formation in AGN hosts. Thus, understanding the excitation of HCN and HCO+ is critical to understanding star formation in both star-forming galaxies and AGN. I will present results from an IRAM 30m survey of HCN (1–0) and HCO+ (1–0) in z<0.09 (U)LIRGs from the Great Observatories All-sky Survey. Using these new data, mid-infrared diagnostics of the AGN contribution to the IR luminosity, and other ancillary data, I will discuss evidence supporting excitation associated with compact gas concentrations, X-ray dominated regions, and mid-infrared pumping, all of which can plausibly enhance emission from HCN and bias dense gas mass determinations.
Title: The Excitation of Dense Molecular Gas Tracers HCN and HCO+ in Local Infrared Luminous Galaxies