Molecular line probes of activity in galaxies

¹ Observatorio de Madrid, OAN, Alfonso XII, 3, 28014-Madrid, Spain
² CfA, SMA project, 60 Garden Street, MS 78 Cambridge, MA 02138, USA

Corresponding author: s.gburillo@oan.es

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 Graciá-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 = 10¹¹L_๏. 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.