HCN versus HCO+ as dense molecular gas mass tracers in luminous infrared galaxies

It has recently been argued that the HCN J = 1 - 0 line emission may not be an unbiased tracer of dense molecular gas (n greater than or similar to 10(4) cm(-3)) in luminous infrared galaxies (LIRGs; L-FIR > 10(11) L circle dot) and that HCO+ J = 1 - 0 may constitute a better tracer instead, casting doubt onto earlier claims supporting the former as a good tracer of such gas. In this paper new sensitive HCN J = 4 - 3 observations of four such galaxies are presented, revealing a surprisingly wide excitation range for their dense gas phase that may render the J = 1 - 0 transition from either species a poor proxy of its mass. Moreover, the well-known sensitivity of the HCO+ abundance to the ionization degree of molecular gas (an important issue omitted from the ongoing discussion about the relative merits of HCN and HCO+ as dense gas tracers) may severely reduce the HCO+ abundance in the star-forming and highly turbulent molecular gas found in LIRGs, while HCN remains abundant. This may result in the decreasing HCO+/HCN J = 1 - 0 line ratios with increasing IR luminosity found in LIRGs, and it casts doubts on HCO+ rather than HCN as a good dense molecular gas tracer. Multi-transition observations of both molecules are needed to identify the best such tracer and its relation to ongoing star formation, and to constrain what may be a considerable range of dense gas properties in such galaxies.