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

R. Meijerink, M. Spaans+1 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Oct 12, 2006

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy31 references217 citations

TL;DR

This paper presents a comprehensive grid of X-ray dominated region (XDR) and photon-dominated region (PDR) models to diagnose irradiated dense gas in galaxy nuclei. By simulating the thermal and chemical balance of molecular gas under varying X-ray and far-ultraviolet irradiation, it identifies key line ratios—such as HCN/HCO⁺, CO(16-15)/CO(1-0), and [SiII]/[CII]—that robustly distinguish XDRs (AGN-dominated) from PDRs (starburst-dominated), even with modest contamination from PDR components.

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.

Motivation & Objective

• To determine the physical conditions in dense molecular gas irradiated by X-rays and far-ultraviolet radiation in active galaxy nuclei.
• To distinguish between XDR (X-ray dominated) and PDR (far-UV dominated) environments using observable line diagnostics.
• To quantify how line ratios and column densities vary with density, irradiation, and column depth to enable observational diagnostics.
• To identify robust line ratios that remain diagnostic despite potential contamination from mixed PDR/XDR components in spatially unresolved observations.
• To guide future observations with Herschel and ALMA by predicting detectable signatures of XDR excitation in high-J CO and molecular lines.

Proposed method

• Numerical modeling of thermal and chemical equilibrium in dense molecular gas (n = 10²–10⁶.⁵ cm⁻³) irradiated by X-rays (1–100 keV) and far-UV (6–13.6 eV).
• Grid-based computation spanning irradiation (10⁰.⁵–10⁵ G₀ and 1.6×10⁻²–160 erg cm⁻² s⁻¹), column density (3×10²¹–1×10²⁵ cm⁻²), and density.
• Incorporation of radiative transfer for atomic fine-structure lines (e.g., [CII] 158 μm, [OI] 63 μm) and rotational lines of molecules (CO, HCN, HNC, CS, etc.) up to J=16.
• Use of collisional excitation and radiative de-excitation rates for molecular line emission, with collisional rate coefficients from literature.
• Comparison of XDR and PDR models under identical total energy input to isolate the effects of ionizing radiation type.
• Calculation of line intensity ratios and column density ratios to identify diagnostic tools for distinguishing XDR and PDR environments.

Experimental results

Research questions

• RQ1 How do atomic fine-structure line ratios, such as [SiII]/[CII] and [OI]/[CII], differ between XDRs and PDRs under comparable irradiation and density?
• RQ2 To what extent do molecular line ratios like HCN/HCO⁺ and HNC/HCN discriminate between XDR and PDR environments, especially at high densities?
• RQ3 How do high-J CO transitions (e.g., CO(16-15)) serve as discriminators between XDR and PDR excitation, and why are they less susceptible to PDR contamination?
• RQ4 What role does column density play in determining the detectability and diagnostic power of HCN, HCO⁺, and other molecular species in XDRs versus PDRs?
• RQ5 Which molecular column density ratios (e.g., CN/HCN, HOC⁺/HCO⁺) are most effective in distinguishing XDR and PDR conditions, and what are their sensitivity thresholds?

Key findings

• For densities >10⁴ cm⁻³, surface temperatures are higher in XDRs than in PDRs due to higher X-ray heating efficiency (up to 70%) compared to FUV (0.5–3.0%).
• The [SiII] 35 μm/[CII] 158 μm line ratio is higher in XDRs than in PDRs for the same density, column, and irradiation, due to differences in ionization balance.
• The HCN(1-0)/HCO⁺(1-0) line ratio is <1 in XDRs and >1 in PDRs when n > 10⁵ cm⁻³ and N_H > 10²³ cm⁻², making it a strong diagnostic at high densities.
• For column densities <10²².⁵ cm⁻², the HCN/HCO⁺ 1-0 ratio in XDRs exceeds unity despite weaker line intensities, indicating a complex dependence on column depth.
• CO(16-15)/CO(1-0) line ratios are significantly higher in XDRs than in PDRs, with differences largest at high-J transitions, making them excellent discriminators even with 10–25% PDR contamination.
• Column density ratios such as CN/HCN, NO/CO, HOC⁺/HCO⁺, and CH/CH⁺ show strong discrimination between XDR and PDR environments, though reliable collisional data are needed for full predictive power.