First CO J = 6–5 and J = 4–3 Detections in Local ULIRGs: The Dense Gas in Markarian 231 and Its Cooling Budget

Abstract

We report on detections of the high-excitation CO J=6-5, J=4-3 lines in Mrk231, 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, 13CO 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(H_2) ~ (1-3)x10^4 cm-3 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 [CII] line at 158 micron, 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 Mrk231 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 cm and mm/sub-mm arrays in detecting CO and HCN transitions in similar systems throughout the local and distant universe.