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 LFIR/LHCN 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 LFIR = 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.