A large CO and HCN line survey of Luminous Infrared Galaxies

A large CO, HCN multi-transition survey of 30 Luminous Infrared Galaxies (${\mathrm{L}}_{\mathrm{I}\mathrm{R}}>{10}^{11}{\mathrm{L}}_{\odot }$) is nearing completion with the James Clerk Maxwell Telescope (JCMT) on Mauna Kea (Hawaii), and the IRAM 30-meter telescope at Pico Veleta (Spain). The CO J=1–0, 2–1, 3–2, 4–3,6–5, ${}^{13}$CO J=2–1, HCN J=1–0, 3–2, 4–3 observations, resulting from $\sim 250$ hours of JCMT, $\sim 100$ hours of 30-m observing time and data from the literature constitute {\it the largest extragalactic molecular line survey to date}, and can be used to address a wide range of issues and eventually allow the construction of reliable Spectral Line Energy Distributions (SLEDs) for the molecular gas in local starbursts. First results suggest that: a) HCN and HCO${}^{+}$ J=1–0 line luminosities can be poor mass estimators of dense molecular gas ($\mathrm{n}\ge {10}^4\mathrm{c}{\mathrm{m}}^{-3}$) unless their excitation is accounted for, b) CO cooling of such gas in ULIRGs may be comparable to that of the CII line at $158\mu \mathrm{m}$, and c) low excitation of the {\it global} molecular gas reservoir remains possible in such systems. In such cases the expected low CO $\mathrm{J}+1\to \mathrm{J}$ line luminosities for $\rm J+1\geq 4$ can lead to a strong bias against their detection from ULIRGs at high redshifts.