Is HCN a true tracer of dense molecular gas in luminous and ultraluminous infrared galaxies?

We present the results of the first HCO+ survey probing the dense molecular gas content of a sample of 16 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN (1-0) observations, had shown that LIRGs and ULIRGs possess a significantly higher fraction of dense molecular gas compared to normal galaxies. While the picture issued from HCO+ partly confirms this result, we have discovered an intriguing correlation between the HCN (1-0)/HCO+ (1-0) luminosity ratio and the IR luminosity of the galaxy (L-IR). This trend casts doubts on the use of HCN as an unbiased quantitative tracer of the dense molecular gas content in LIRGs and ULIRGs. A plausible scenario explaining the observed trend implies that X-rays coming from an embedded active galactic nucleus may play a dominant role in the chemistry of molecular gas at L-IR >= 10(12) L-circle dot. We discuss the implications of this result for the understanding of LIRGs, ULIRGs, and high- redshift gas- rich galaxies.