We report the detection of far-infrared ( FIR ) CO rotational emission from nearby active galactic nuclei ( AGN ) and starburst galaxies , as well as several merging systems and Ultra-Luminous Infrared Galaxies ( ULIRGs ) .
Using Herschel-PACS , we have detected transitions in the J _ { upp } = 14 - 20 range ( \lambda \sim 130 - 185 \mu m , \nu \sim 1612 - 2300 GHz ) with upper limits on ( and in two cases , detections of ) CO line fluxes up to J _ { upp } = 30 .
The PACS CO data obtained here provide the first well-sampled FIR extragalactic CO Spectral Line Energy Distributions ( SLEDs ) for this range , and will be an essential reference for future high redshift studies .
We find a large range in the overall SLED shape , even amongst galaxies of similar type , demonstrating the uncertainties in relying solely on high-J CO diagnostics to characterize the excitation source of a galaxy .
Combining our data with low-J line intensities taken from the literature , we present a CO ratio-ratio diagram and discuss its potential diagnostic value in distinguishing excitation sources and physical properties of the molecular gas .
The position of a galaxy on such a diagram is less a signature of its excitation mechanism , than an indicator of the presence ( or absence ) of warm , dense molecular gas .
We then quantitatively analyze the CO emission from a subset of the detected sources with Large Velocity Gradient ( LVG ) radiative transfer models to fit the CO SLEDs .
Using both single-component and two-component LVG models to fit the kinetic temperature , velocity gradient , number density and column density of the gas , we derive the molecular gas mass and the corresponding CO-to-H _ { 2 } conversion factor , \alpha _ { CO } , for each respective source .
For the ULIRGs we find \alpha values in the canonical range 0.4 - 5 M _ { \odot } / ( K kms ^ { -1 } pc ^ { 2 } ) , while for the other objects , \alpha varies between 0.2 and 14 .
Finally , we compare our best-fit LVG model results with those obtained in previous studies of the same galaxies and comment on any differences .