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 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 ( ⟨ ?> L HCN ( 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(L IR), consistent with most previous findings. L HCN ( 1 – 0 ) ′ ?> /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 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.