accretion disks, and the broad emission line regions in active galactic nuclei.'
author:
- 'D. Nikolić, T. W. Gorczyca, K. T. Korista'
- 'G. J. Ferland'
- 'N. R. Badnell'
title: Suppression of Dielectronic Recombination Due to Finite Density Effects
---

Introduction {#intro}
============

Astronomical emission or absorption sources have an enormous range of densities. Two examples include the intergalactic medium, with $n_{\rm{e}} \sim 10^{-4}\ {\rm cm}^{-3}$, and the broad emission-line regions of Active Galactic Nuclei, with $n_{\rm{e}} \sim 10^{10}\ {\rm cm}^{-3}$. The gas producing the spectrum is not in thermodynamic equilibrium [@osterbrockferland], so microphysical processes determine the physical conditions.

The two common cases encountered for ionization are photoionization and collisional (e.g., electron-impact) ionization. In both cases, ions are recombined by dielectronic and radiative recombination, with dielectronic recombination (DR) usually the dominant process for elements heavier than helium. Databases give ionization and recombination rates that are the sum of several contributing processes. Examples include @voronov for electron impact ionization, [@verner] for photoionization, and the DR project [@drproject] for dielectronic recombination and [@rrproject] for radiative recombination; it is these latter data [^1] which will be of primary interest to us in the present study.

The collisional ionization and recombination rate coefficients 