| Description |
One of the most exciting first results from SPIRE and PACS spectroscopy is the detection of high-J CO emission in galaxies. Such emission has long been proposed as a tracer of X-ray dominated regions (XDRs) produced by AGN, and as a powerful diagnostic tool for future millimeter-wave study of AGN at high redshift. The shortest wavelength submillimeter CO lines detected by SPIRE-FTS in the X-ray luminous ULIRG Mrk 231 are interpreted as arising from an extended XDR, providing strong observational support for the XDR/high-J CO connection. However, our group (SHINING; PI E. Sturm) has used PACS to detect even higher-J far-IR CO emission in a few nearby galaxies, including 2 starburst galaxies with little evidence of a luminous AGN. Can high-J 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 far-IR CO lines in 4 nearby AGN and 4 starburst galaxies, as well as in 2 merging systems with large masses of shock-heated molecular gas. Does the high-J 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 X-ray-driven 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 far-IR fine-structure lines, which includes important tracers of UV-, X-ray-, and shock-heated gas. The AGN subsample is restricted to the most nearby systems, where the high spatial resolution of PACS is sufficient to separate the nuclear AGN-heated gas from the circumnuclear star-forming regions. The PACS CO data obtained here will provide the first well-sampled far-IR extragalactic CO line SEDs, and will be an essential reference for future high redshift studies. |
