Evidence of enhanced star formation efficiency in luminous and ultraluminous infrared galaxies

We present new observations made with the IRAM 30 m telescope of the J=1 - 0 and 3 - 2 lines of HCN and HCO+ used to probe the dense molecular gas content in a sample of 17 local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). These observations have allowed us to derive an updated version of the power law describing the correlation between the FIR luminosity (L-FIR) and the HCN(1 - 0) luminosity (LHCN(1 - 0)') of local and high-redshift galaxies. We present the first clear observational evidence that the star formation efficiency of the dense gas (SFEdense), measured as the L-FIR/LHCN(1 - 0)' ratio, is significantly higher in LIRGs and ULIRGs than in normal galaxies, a result that has also been found recently in high-redshift galaxies. This may imply a statistically significant turn upward in the Kennicutt-Schmidt law derived for the dense gas at L-FIR >= 10(11) L-circle dot. We used a one-phase large velocity gradient (LVG) radiative transfer code to fit the three independent line ratios derived from our observations. The results of this analysis indicate that the [HCN]/[HCO+] abundance ratios could be up to one order of magnitude higher than normal in a significant number of the LIRGs and ULIRGs in our sample. An overabundance of HCN at high LFIR implies that the reported trend in the L-FIR/L-HCN' ratio as a function of LFIR would be underestimating a potentially more dramatic change of the SFEdense. Results obtained with two-phase LVG models corroborate that the LHCN(1 - 0)'-to-M-dense conversion factor must be lowered at high L-FIR. We discuss the implications of these findings for the use of HCN as a tracer of the dense molecular gas in local and high-redshift luminous infrared galaxies.