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Radiation pressure confinement - II. Application to the broad line region in active galactic nuclei | | Alexei Baskin
; Ari Laor
; Jonathan Stern
; | | Date: |
30 Sep 2013 | | Abstract: | Active galactic nuclei (AGN) are characterized by broad emission lines. The
lines show similar properties from the lowest luminosity (10^39 erg/s) to the
highest luminosity (10^47 erg/s) AGN. What produces this similarity over such a
vast range of 10^8 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 clouds with a large enough column for gravity to
dominate. The photoionized surface layer of these clouds must develop a
pressure gradient which balances 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,
set by the ambient pressure, a density rise inwards, and a uniform density
cooler inner region, where the gas pressure, 2n_ekT, reaches the incident
radiation pressure n_gamma<hnu>, where n_e is the electron density, n_gamma is
the photon density, and <hnu> is the mean photon energy. This radiation
pressure confinement (RPC) of the photoionized layer leads to a universal
ionization parameter U=n_gamma/n_e~0.1 in the inner photoionized layer,
independent of distance from the AGN, and of the AGN luminosity. 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, which are the only free parameters. The predicted mean
strength of most lines, including O VI 1034 and He II 1640 (but excluding H
beta), and their peak emissivity radius, are consistent with observations. | | Source: | arXiv, 1309.7953 | | Services: | Forum | Review | PDF | Favorites | |
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