2007ApJ...668..815P

We report on detections of the high-excitation CO J=6-5 and J=4-3 lines in Mrk 231, a prototypical ultraluminous infrared galaxy (ULIRG) and Seyfert 1 QSO. These observations are combined with CO J=3-2 and HCN J=4-3 measurements from this work and CO J=2-1, CO J=1-0, ¹³CO J=2-1, and 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)x10⁴/cm3 and T_k∼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 158 µm, 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 well-constrained spectral line energy distributions (SLEDs) for the dense molecular gas in Mrk 231 and compare them to those of high-redshift starbursts, many of which have SLEDs that may be affected by strong lensing. Finally, we use our local molecular line excitation template to assess the capabilities of future centimeter and millimeter/submillimeter arrays in detecting CO and HCN transitions in similar systems throughout the local and distant universe.