Radiation pressure confinement - II. application to the broad-line region in active galactic nuclei

Alexei Baskin*, Ari Laor, Jonathan Stern

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

62 Scopus citations


Active galactic nuclei (AGN) are characterized by similar broad emission lines properties at all luminosities (1039 - 1047 erg s-1). What produces this similarity over a vast range of 108 in luminosity? Photoionization is inevitably associated with momentum transfer to the photoionized gas. Yet, most of the photoionized gas in the broad-line region (BLR) follows Keplerian orbits, which suggests that the BLR originates from gas with a large enough column for gravity to dominate. The photoionized surface layer of the gas must develop a pressure gradient due to the incident radiation force. We present solutions for the structure of such a hydrostatic photoionized gas layer in the BLR. The gas is stratified, with a low-density highly ionized surface layer, a density rise inwards and a uniform-density cooler inner region, where the gas pressure reaches the incident radiation pressure. This radiation pressure confinement (RPC) of the photoionized layer leads to a universal ionization parameter U ̃ 0.1 in the inner photoionized layer, independent of luminosity and distance. Thus, RPC appears to explain the universality of the BLR properties in AGN. We present predictions for the BLR emission per unit covering factor, as a function of distance from the ionizing source, for a range of ionizing continuum slopes and gas metallicity. The predicted mean strength of most lines (excluding H β), and their different average-emission radii, are consistent with available observations.

Original languageEnglish
Pages (from-to)604-619
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Feb 2014
Externally publishedYes


  • Galaxies:Active
  • Quasars:Emission lines
  • Quasars:general


Dive into the research topics of 'Radiation pressure confinement - II. application to the broad-line region in active galactic nuclei'. Together they form a unique fingerprint.

Cite this