Diagnostics of irradiated gas in galaxy nuclei

I. A far-ultraviolet and X-ray dominated region code

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: meijerin@strw.leidenuniv.nl
² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 19 November 2004
Accepted: 22 February 2005

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 ~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.