Testing the XDR/High-J CO Paradigm in Nearby Galaxies

One of the most exciting first results from SPIRE and PACS spectroscopy is the detection of highJ CO emission in galaxies. Such emission has long been proposed as a tracer of Xray dominated regions (XDRs) produced by AGN, and as a powerful diagnostic tool for future millimeterwave study of AGN at high redshift. The shortest wavelength submillimeter CO lines detected by SPIREFTS in the Xray luminous ULIRG Mrk 231 are interpreted as arising from an extended XDR, providing strong observational support for the XDR/highJ CO connection. However, our group (SHINING; PI E. Sturm) has used PACS to detect even higherJ farIR CO emission in a few nearby galaxies, including 2 starburst galaxies with little evidence of a luminous AGN. Can highJ CO emission also be produced in gas heated by the UV radiation or mechanical output of a starburst? To address this question we propose to measure a set of farIR CO lines in 4 nearby AGN and 4 starburst galaxies, as well as in 2 merging systems with large masses of shockheated molecular gas. Does the highJ CO line SED reflect the different excitation mechanisms in these template objects? We additionally propose to use OH line observations to estimate the abundance of this molecule, which is a sensitive tracer of Xraydriven chemistry. The OH line profiles will also be used to search for evidence of molecular outflows, which may drive shock heating. Each of these galaxies will have SHINING GT observations of the set of farIR finestructure lines, which includes important tracers of UV, Xray, and shockheated gas. The AGN subsample is restricted to the most nearby systems, where the high spatial resolution of PACS is sufficient to separate the nuclear AGNheated gas from the circumnuclear starforming regions. The PACS CO data obtained here will provide the first wellsampled farIR extragalactic CO line SEDs, and will be an essential reference for future high redshift studie truncated!, Please see [truncated!, Please see actual data for full text]