2006ApJ...640L.135G

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≥10¹² L_☉. We discuss the implications of this result for the understanding of LIRGs, ULIRGs, and high-redshift gas-rich galaxies.