The ISM of active galaxy centers is exposed to a combination of cosmic ray , FUV and X-ray radiation .
We apply PDR models to this ISM with both ‘ normal ’ and highly elevated ( 5 \times 10 ^ { -15 } s ^ { -1 } ) cosmic-ray rates and compare the results to those obtained for XDRs .
Our existing PDR-XDR code is used to construct models over a 10 ^ { 3 } -10 ^ { 5 } cm ^ { -3 } density range and for 0.16-160 erg s ^ { -1 } cm ^ { -2 } impingent fluxes .
We obtain larger high J ( J > 10 ) CO ratios in PDRs when we use the highly elevated cosmic ray rate , but these are always exceeded by the corresponding XDR ratios .
The [ CI ] 609 \mu m/ ^ { 13 } CO ( 2-1 ) line ratio is boosted by a factor of a few in PDRs with n \sim 10 ^ { 3 } cm ^ { -3 } exposed to a high cosmic ray rate .
At higher densities ratios become identical irrespective of cosmic ray flux , while XDRs always show elevated [ CI ] emission per CO column .
The HCN/CO and HCN/HCO ^ { + } line ratios , combined with high J CO emission lines , are good diagnostics to distinguish between PDRs under either low or high cosmic ray irradiation conditions , and XDRs .
Hence , the HIFI instrument on Herschel , which can detect these CO lines , will be crucial in the study of active galaxies .