Molecular line probes of activity in galaxies

TL;DR

This paper investigates how specific molecular line tracers reveal the impact of galactic activity on dense interstellar gas, showing that star formation efficiency is higher in luminous infrared galaxies, affecting how we interpret molecular observations.

Contribution

It provides the first clear observational evidence that star formation efficiency in dense gas is significantly higher in LIRGs and ULIRGs compared to normal galaxies, with implications for molecular gas tracers.

Findings

Star formation efficiency is higher in LIRGs/ULIRGs.

Overabundance of HCN in active environments.

Implication for using HCN as a dense gas tracer.

Abstract

The use of specific tracers of the dense molecular gas phase can help to explore the feedback of activity on the interstellar medium (ISM) in galaxies. This information is a key to any quantitative assessment of the efficiency of the star formation process in galaxies. We present the results of a survey devoted to probe the feedback of activity through the study of the excitation and chemistry of the dense molecular gas in a sample of local universe starbursts and active galactic nuclei (AGNs). Our sample includes also 17 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007) we find the first clear observational evidence that the star formation efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting evidence of overabundant HCN in active environments would even reinforce the reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt law around L_FIR=10^11 L_sun. This result has major implications for the use of HCN as a tracer of the dense gas in local and high-redshift luminous infrared galaxies.