[Paper Review] Diagnostics of irradiated gas in galaxy nuclei. I: A Far-ultraviolet and X-ray dominated region code

R. Meijerink, M. Spaans|ArXiv.org|Feb 22, 2005

Astrophysics and Star Formation StudiesPhysics and Astronomy56 references254 citations

TL;DR

This paper presents a coupled far-ultraviolet (PDR) and X-ray dominated region (XDR) code to model irradiated gas in galaxy nuclei, incorporating detailed thermal, chemical, and radiative processes including PAHs and H₂ excitation. Key results show that XDR column density ratios (e.g., CO/C, HNC/HCN) remain nearly constant up to $N_{ m H} = 10^{22} m hinspace cm^{-2}$, unlike PDRs where they vary dramatically, highlighting fundamental differences in chemical structure between PDRs and XDRs under varying radiation fields and densities.

ABSTRACT

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 ~10^20 and ~10^25 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 N_H=10^22 cm^-2 for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to N_H=10^22 cm^-2. The CO/C and CO/H2 ratios are lower in XDRs at low column densities and rise at N_H > 10^23 cm^-2. At most column densities N_H > 10^21.5 cm^-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N_H.

Motivation & Objective

• To develop a comprehensive PDR and XDR code that models irradiated gas in galaxy nuclei under realistic physical conditions.
• To investigate the thermal and chemical structure of gas exposed to far-ultraviolet (FUV) and X-ray radiation fields.
• To compare PDR and XDR physics across a range of densities and radiation fields relevant to starburst galaxies and active galactic nuclei.
• To provide a diagnostic tool for interpreting observed fine-structure lines and column density ratios in high-redshift and local galaxy nuclei.
• To quantify how column density ratios such as CO/C, HNC/HCN, and CO/H₂ evolve with increasing total hydrogen column density in both PDR and XDR environments.

Proposed method

• The model uses a semi-infinite slab geometry irradiated from one side with no geometrical dilution, simulating conditions in galaxy centers.
• An elaborate chemical network is employed, including ion-molecule reactions, photoionization, and charge transfer processes driven by FUV and X-ray fields.
• Thermal balance is calculated by including heating from photoelectric emission (dust and PAHs), FUV pumping of H₂, and cosmic rays, and cooling via fine-structure lines and molecular rotational transitions.
• H₂ formation, destruction, and vibrational excitation are modeled using rate coefficients from Tine et al. (1997) and Yan (1997), including statistical distribution of vibrational energy upon formation.
• X-ray absorption is calculated using total elemental abundances and X-ray cross sections from Verner & Yakovlev (1995), with depth-dependent flux attenuation via $F(E,z) = F(E,0) imes m exp(- au)$.
• Line intensities and column densities are computed by integrating over depth, with radiative transfer handled through self-consistent ionization and excitation calculations.

Experimental results

Research questions

• RQ1 How do column density ratios such as CO/C, HNC/HCN, and CO/H₂ evolve with increasing total hydrogen column density $N_{ m H}$ in PDRs versus XDRs?
• RQ2 What is the impact of FUV versus X-ray radiation fields on the thermal structure and chemical composition of irradiated gas in galaxy nuclei?
• RQ3 How do the cumulative line intensities of key diagnostic lines like [CII] 158 μm, [OI] 63 μm, and H₂ ro-vibrational lines differ between PDR and XDR models?
• RQ4 To what extent do XDRs maintain constant abundance ratios over a wide range of column densities compared to PDRs?
• RQ5 How do the relative abundances of carbon species (C⁺, C, CO) change across the cloud depth in PDRs and XDRs under varying radiation fields?

Key findings

• CO/C column density ratios in PDRs vary by over four orders of magnitude from the cloud surface to $N_{ m H} = 10^{22} m hinspace cm^{-2}$, indicating strong sensitivity to radiation field and density.
• In XDRs, CO/C ratios remain nearly constant up to $N_{ m H} = 10^{22} m hinspace cm^{-2}$, suggesting a more stable chemical structure under X-ray irradiation.
• CO/C and CO/H₂ ratios are lower in XDRs at low column densities but increase significantly at $N_{ m H} > 10^{23} m hinspace cm^{-2}$, indicating a transition to enhanced molecular formation in deep XDRs.
• HNC/HCN ratios are lower in XDRs than in PDRs at $N_{ m H} > 10^{21.5} m hinspace cm^{-2}$, but show a more moderate increase with increasing column density compared to PDRs.
• Fine-structure line intensities such as [CII] 158 μm and [OI] 63 μm are strong in both PDRs and XDRs, but their relative strengths differ due to distinct ionization and excitation mechanisms.
• The model predicts that XDRs exhibit smoother transitions from H to H₂ and from C⁺ to C to CO, consistent with the volume-dominated nature of X-ray heating compared to the surface-dominated FUV heating in PDRs.