Irradiated ISM: Discriminating between Cosmic Rays and X-Rays

The interstellar medium (ISM) at the centers of active galaxies is exposed to a combination of cosmic-ray, far-ultraviolet (FUV), and X-ray radiation. We apply photodissociation region (PDR) models to this ISM with both "normal" and highly elevated (5 × 10-15 s-1) cosmic-ray (CR) rates and compare the results to those obtained for X-ray dissociation regions (XDRs). Our existing PDR-XDR code is used to construct models over a 103-105 cm-3 density range and for 0.16-160 ergs s-1 cm-2 impingent fluxes. We obtain larger high-J (J > 10) CO ratios in PDRs when we use the highly elevated CR rate, but these are always exceeded by the corresponding XDR ratios. The [C I] 609 μm/13CO (2-1) line ratio is boosted by a factor of a few in PDRs with n ~ 103 cm-3 exposed to a high CR rate. At higher densities, ratios become identical irrespective of CR flux, while XDRs always show elevated [C I] 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 CR irradiation conditions, and XDRs. Hence, the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory, which can detect these CO lines, will be crucial in the study of active galaxies.