Irradiated ISM: Discriminating between Cosmic Rays and X-Rays

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' andhighly elevated (5\times 10^{-15}s^-1) cosmic-ray rates and compare the resultsto those obtained for XDRs. Our existing PDR-XDR code is used to constructmodels over a 10^3-10^5 cm^-3 density range and for 0.16-160 erg s^-1 cm^-2impingent fluxes. We obtain larger high J (J>10) CO ratios in PDRs when we usethe highly elevated cosmic ray rate, but these are always exceeded by thecorresponding XDR ratios. The [CI] 609 mum/13CO(2-1) line ratio is boosted by afactor of a few in PDRs with n~10^3 cm^-3 exposed to a high cosmic ray rate. Athigher densities ratios become identical irrespective of cosmic ray flux, whileXDRs 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 todistinguish between PDRs under either low or high cosmic ray irradiationconditions, and XDRs. Hence, the HIFI instrument on Herschel, which can detectthese CO lines, will be crucial in the study of active galaxies.Comment: accepted by Astrophysical Journal Letter