EXCITATION MECHANISMS FOR HCN(1–0) AND HCO⁺(1–0) IN GALAXIES FROM THE GREAT OBSERVATORIES ALL-SKY LIRG SURVEY*

Abstract

ABSTRACT We present new Institut de Radioastronomie Millimétrique (IRAM) 30 m spectroscopic observations of the ∼88 GHz band, including emission from the multiplet, HCN ( , , and , 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 ( / ), compared to composite and starburst-dominated systems ( / and 0.88, respectively). However, some composite and starburst systems have / 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 log 10 ( ) and log 10 ( L IR ), consistent with most previous findings. / L IR , typically interpreted as the dense-gas depletion time, appears to have no systematic trend with L IR for our sample of luminous and ultraluminous infrared galaxies, and has significant scatter. The galaxy-integrated and 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.