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

TL;DR

This paper introduces detailed PDR and XDR models for irradiated gas in galaxy nuclei, comparing their physical and chemical properties across various conditions relevant to starburst galaxies and active galactic nuclei.

Contribution

It provides comprehensive models for PDR and XDR regions, including chemical networks and line predictions, and offers a detailed comparison of their physical and chemical characteristics.

Findings

Column density ratios are nearly constant in XDRs up to N_H=10^22 cm^-2.

CO/C ratio varies significantly in PDRs but remains stable in XDRs at certain depths.

HNC/HCN ratios differ between XDRs and PDRs, with distinct behaviors at high column 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.