We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H2 formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between ~1020 and ~1025 cm-2. We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species (e.g., CO/H2, CO/C, HCO+/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to . The CO/C and CO/H2 ratios are lower in XDRs at low column densities and rise at . At most column densities , HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N H.

Aims.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.Methods.These models iteratively determine the thermal and chemical balance of molecular gas that is exposed to X-rays (1–100 keV) and far-ultraviolet radiation (6–13.6 eV), as a function of depth.Results. We present a grid of XDR and PDR models that span ranges in density ( cm-3), irradiation ( and erg cm-2 s-1) and column density ( cm-2). Predictions are made for the most important atomic fine-structure lines, e.g., [CII], [OI], [CI], [SiII], and for molecular species like HCO+, HCN, HNC, CS and SiO up to , CO and 13CO up to , and column densities for CN, CH, CH+, HCO, HOC+, NO and N2H+. We find that surface temperatures are higher (lower) in PDRs compared to XDRs for densities >104 (<104) cm-3. For the atomic lines, we find that, largely due to the different XDR ionization balance, the fine-structure line ratios of [SiII] 35 μm/[CII] 158 μm, [OI] 63 μm/[CII] 158 μm, [FeII] 26 μm/[CII] 158 μm and [CI] 369 μm/[CI] 609 μm are larger in XDRs than in PDRs, for a given density, column and irradiation strength. Similarly, for the molecular lines, 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. In particular, the HCN/HCO+ 1–0 ratio is 1) for XDRs (PDRs) if the density exceeds 105 cm-3 and if the column density is larger than 1023 cm-2. For columns less than 1022.5 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, cm-3, and strong radiation fields (>100 erg s-1 cm-2), HCN/HCO+ ratios can become larger in XDRs than PDRs as well. Also, 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 high (column) density gas, with columns in excess of 1023 cm-2 and densities larger than 104 cm-3. Furthermore, CO is typically warmer in XDRs than in PDRs, for the same total energy input. This leads to higher CO /CO 1–0, , line ratios in XDRs. In particular, lines with , like CO(16–15) and CO(10–9) observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. This is crucial since the XDR/AGN contribution will typically be of a much smaller (possibly beam diluted) angular scale and a 10–25% PDR contribution can already suppress XDR distinguishing features involving HCN/HCO+ and HNC/HCN. For possible future observations, column density ratios indicate that CH, CH+, NO, HOC+ and HCO are good PDR/XDR discriminators.

Aims.We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution.Methods. A number of benchmark models have been created, covering low and high gas densities cm-3 and far ultraviolet intensities χ = 10, 105 in units of the Draine field (FUV: 6 < < 13.6 eV). The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature self consistently by solving the thermal balance, thus testing the modeling of the heating and cooling mechanisms accounting for the detailed energy balance throughout the clouds.Results.We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes such as the effect of PAHs on the electron density or the temperature dependence of the dissociation of CO by cosmic ray induced secondary photons, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly. Nevertheless, the remaining spread in the computed observables such as the atomic fine-structure line intensities serves as a warning that there is still a considerable uncertainty when interpreting astronomical data with our models.

We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk 231. In total 25 lines are detected, including CO J = 5–4 through J = 13–12, 7 rotational lines of H2O, 3 of OH+ and one line each of H2O+, CH+, and HF. We find that the excitation of the CO rotational levels up to J = 8 can be accounted for by UV radiation from star formation. However, the approximately flat luminosity distribution of the CO lines over the rotational ladder above J = 8 requires the presence of a separate source of excitation for the highest CO lines. We explore X-ray heating by the accreting supermassive black hole in Mrk 231 as a source of excitation for these lines, and find that it can reproduce the observed luminosities. We also consider a model with dense gas in a strong UV radiation field to produce the highest CO lines, but find that this model strongly overpredicts the hot dust mass in Mrk 231. Our favoured model consists of a star forming disk of radius 560 pc, containing clumps of dense gas exposed to strong UV radiation, dominating the emission of CO lines up to J = 8. X-rays from the accreting supermassive black hole in Mrk 231 dominate the excitation and chemistry of the inner disk out to a radius of 160 pc, consistent with the X-ray power of the AGN in Mrk 231. The extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of X-ray driven excitation and chemistry in this region.