HCN versus HCO as dense molecular gas mass tracer in Luminous Infrared Galaxies

It has been recently argued that the HCN J=1–0 line emission may not be an unbiased tracer of dense molecular gas (n & 10 cm) in Luminous Infrared Galaxies (LIRGs: LFIR > 10 11 L⊙) and HCO + J=1–0 may constitute a better tracer instead (Graciá-Carpio et al. 2006), casting doubt into earlier claims supporting the former as a good tracer of such gas (Gao & Solomon 2004; Wu et al. 2006). 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 on the ionization degree of the 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 to the decreasing HCO/HCN J=1–0 line ratio with increasing IR luminosity found in LIRGs, and casts doubts on the HCO rather than the HCN as a good dense molecular gas tracer. Multi-transition observations of both molecules are needed to identify the best such tracer, its relation to ongoing star formation, and constrain what may be a considerable range of dense gas properties in such galaxies. Subject headings: galaxies: starbursts – galaxies: active – ISM: molecules – ISM: HCN, HCO – radio lines: galaxies