  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 posses 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 AGN may play a dominant role in the chemistry of molecular gas at
L(IR) > 1e12 Lsun. We discuss the implications of this result for the
understanding of LIRGs, ULIRGs and high redshift gas-rich galaxies.
