Diagnostics of irradiated dense gas in galaxy nuclei. II. A grid of XDR and PDR models

Abstract

\abridged The nuclei of active galaxies harbor massive young stars, an accreting central black hole, or both. In order to determine the physical conditions that pertain to molecular gas close to the sources of radiation, numerical models are constructed. These models determine the thermal and chemical balance of molecular gas that is exposed to X-rays and far-ultraviolet radiation, as a function of depth. We present a grid of XDR and PDR models that span ranges in density, irradiation and column density. We find that the fine-structure line ratios of e.g. [SiII] 35mum/[CII] 158 mum are larger in XDRs than in PDRs, for a given density, column and irradiation strength. We find that the line ratios HCN/HCO+ and HNC/HCN, as well as the column density ratio CN/HCN, discriminate between PDRs and XDRs. The HCN/HCO+ 1-0 ratio is <1 (>1) for XDRs (PDRs) if the density exceeds 10^5 cm^-3 and if the column density is >10^23 cm^-2. For columns <10^ 22.5 cm^-2 the XDR HCN/HCO+ 1-0 ratio becomes larger than one, although the individual HCN 1-0 and HCO+ 1-0 line intensities are weaker. For modest densities, n=10^4-10^5 cm^-3, and strong radiation fields (>100 erg s^-1 cm^-2), HCN/HCO+ ratios can become larger in XDRs than PDRs as well. The HCN/CO 1-0 ratio is typically smaller in XDRs, and the HCN emission in XDRs is boosted with respect to CO only for columns >10^{23} cm^{-2} and densities <10^4 cm^-3. CO is typically warmer in XDRs than in PDRs, for the same total energy input. This leads to higher CO J=N+1-N/CO 1-0, N>=1, line ratios in XDRs. Lines with N>=10, like CO(16-15) and CO(10-9) observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. Column density ratios indicate that CH, CH+, NO, HOC+ and HCO are good PDR/XDR discriminators.