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 x 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 10(3)-10(5) 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 mu m/(13)CO (2-1) line ratio is boosted by a factor of a few in PDRs with n similar to 10(3) 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.