Excitation Mechanisms for HCN (1-0) and HCO+ (1-0) in Galaxies from the Great Observatories All-sky LIRG Survey

November 2015 • 2015ApJ...814...39P

Authors • Privon, G. C. • Herrero-Illana, R. • Evans, A. S. • Iwasawa, K. • Perez-Torres, M. A. • Armus, L. • Díaz-Santos, T. • Murphy, E. J. • Stierwalt, S. • Aalto, S. • Mazzarella, J. M. • Barcos-Muñoz, L. • Borish, H. J. • Inami, H. • Kim, D. -C. • Treister, E. • Surace, J. A. • Lord, S. • Conway, J. • Frayer, D. T. • Alberdi, A.

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\to 0) multiplet, HCN (J=1\to 0), {{HCO}}+(J=1\to 0), and {HNC}(J=1\to 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)}\prime /{L}{{HCO}+(1-0)}\prime > =1.84), compared to composite and starburst-dominated systems (< {L}{HCN(1-0)}\prime /{L}{{HCO}+(1-0)}\prime > =1.14, and 0.88, respectively). However, some composite and starburst systems have {L}{HCN(1-0)}\prime /{L}{{HCO}+(1-0)}\prime 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)}\prime ) and log10(LIR), consistent with most previous findings. {L}{HCN(1-0)}\prime /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 galaxy-integrated {L}{HCN(1-0)}\prime and {L}{{HCO}+(1-0)}\prime 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.

Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).


IPAC Authors


Lee Armus

Senior Scientist

Joe Mazzarella

Senior Scientist