CO, [CI] and [NII] lines from Herschel spectra

This work examines the relationships between L_FIR_, L'_CO_, and L_CO_/L_CO,1-0_. We also present a new method for estimating probability distribution functions from marginal signal-to-noise ratio Herschel FTS spectra, which takes into account the instrumental "ringing" and the resulting highly correlated nature of the spectra.

The slopes of log(L_FIR_) versus log(L'_CO_) are linear for all mid- to high-J CO lines and slightly sublinear if restricted to (ultra)luminous infrared galaxies ((U)LIRGs). The mid- to high-J CO luminosity relative to CO J=1-0 increases with increasing L_FIR_, indicating higher excitement of the molecular gas, although these ratios do not exceed ~180. For a given bin in L_FIR_, the luminosities relative to CO J=1-0 remain relatively flat from J=6-5 through J=13-12, across three orders of magnitude of L_FIR_.