Molecular gas in extreme star-forming environments: the starbursts Arp220 and NGC6240 as case studies

We report single-dish observations of 12CO, 13CO, C18O, HCN, HNC, HNCO, CS, and HCO+ molecular line emission in the two ultraluminous infra-red galaxies Arp220 and NGC6240. Using this new molecular line inventory in conjunction with existing data in the literature, we have compiled the most extensive molecular line data sets to date of such galaxies. These include rotational transitions of molecules with very different excitation requirements, thus allowing us to probe molecular gas phases with widely different densities and temperatures. In particular, we obtain strong constraints on the properties of the dense gas phase in Arp220 and NGC6240, from which we obtain best estimates of the dense (>10^4 cm^-3) gas mass in these two systems of M_dense = (0.6 - 2.0) x 10^10 Msun and M_dense = (0.8 - 2.4) x 10^10 Msun, respectively. Such large dense gas masses suggest that the bulk of the total gas reservoir in Arp220 and NGC6240 is in the dense phase -- consistent with the notion that most of their IR luminosities are powered by intense star bursts. Our analysis suggest that HCO+/HCN/CS probe progressively denser gas (by factors of ten), consistent with the dense phase residing in a hierarchical structure with denser gas nested inside less dense regions. In particular, we find, contrary to recent claims, that HCN is a better tracer of dense gas than HCO+, which is sub-thermally excited in both systems, thus supporting the notion that HCN can be used to derive reliable dense gas masses and star formation efficiencies. However, whether this is true in IR-luminous, star forming galaxies in general, remains to be seen, and underlines the need for high-J, high-dipole observations of large samples of low- to moderate-z ULIRGs -- a task which we show the HI-FI instrument on board Herschel is ideally suited to do