J un 2 00 7 First CO J = 6 – 5 , 4 – 3 detections in local ULIRGs : the dense gas in Mrk 231 , and its cooling budget

We report on detections of the high-excitation CO J=6–5, J=4–3 lines in Mrk 231, a prototypical Ultra Luminous Infrared Galaxy (ULIRG) and Seyfert 1 QSO. These observations are combined with CO J=3–2, HCN J=4–3 (this work), and CO J=2–1, J=1–0, CO J=2–1, HCN J=1–0 measurements taken from the literature to provide better constraints on the properties of the molecular gas in an extreme starburst/QSO in the local Universe. We find that the CO J=4–3 and J=6–5 transitions trace a different gas phase from that dominating the lower three CO transitions, with n(H2) ∼ (1 − 3) × 10 cm and Tk ∼ (40 − 70)K. This phase is responsible for the luminous HCN emission, and contains most of the H2 gas mass of this galaxy. The total CO line cooling emanating from this dense phase is found similar to that of the [C ii] line at 158μm, suggesting a very different thermal balance to that seen in lower IR-luminosity galaxies, and one likely dominated by dense photon-dominated regions. Our dense “sampling” of the CO rotational ladder and the HCN lines enables us to produce wellconstrained Spectral Line Energy Distributions (SLEDs) for the dense molecular