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

Abstract

Active galactic nuclei (AGN) are characterized by similar broad emission lines properties at all luminosities (10 39 -10 47 erg s −1 ). What produces this similarity over 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 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 the available observations.

The thermal instability of the warm absorber in NGC 3783

“…6 (bottom) showing P rad /P gas = P tot /P gas − 1 that the radiative pressure dominates almost across the entire slab and is nearly constant except at the very far side where it drops sharply. This is in good agreement with the assumption of a RPC wind as discussed in Baskin et al (2014) and Stern et al (2014a,b).…”

“…Thereby, the radiation pressure largely dominates compared to the gas pressure. This suggests that the two or three absorbing clouds are confined by the radiation pressure, a model recently explored by Stern et al (2014a,b) and Baskin et al (2014) for the line emitting regions in active galactic nuclei. A radiation pressure compressed (RPC) wind can be realized by assuming a photo-ionized medium that is isobaric in terms of the total (gas+radiation) pressure.…”

The mass and spin of the extreme Narrow Line Seyfert 1 Galaxy 1H 0707−495 and its implications for the trigger for relativistic jets

“…The radiation pressure correction to the mass is not widely used as the general lack of observed wind features for the low ionisation BLR lines form a strong argument against such effects being important (Baskin, Laor & Stern 2014a). Physically, this could indicate that the clouds are optically thick to electron scattering, with columns of > 10 24 cm −2 (consistent with the low ionisation broad lines forming in the disc: Collin-Souffrin et al 1980), or that the radiative acceleration only affects the front face of the cloud (Baskin, Laor & Stern 2014b).…”

On the origins of C iv absorption profile diversity in broad absorption line quasars

“…As the SED becomes softer (lower He ii EW), the production of C 3+ decreases for a radiationbounded gas, and the C iv emission EW becomes lower (e.g. Baskin, Laor & Stern 2014a). Radiation-bounded gas absorbs all of the ionizing radiation, and thus the line emission should be stronger for a harder SED that has more ionizing photons, as observed (Fig.…”

“…They adopt a baseline of 0.1 for the BLR CF, and use the Locally Optimally emitting Cloud (LOC) model of the BLR, where a single slab of gas produces line emission from a rather limited range of ionization states (Baldwin et al 1995). However, adopting the observed BLR CF of ≃0.3 (see above), and noting that a single slab produces line emission from a very broad range of ionization states (Baskin et al 2014a), alleviate the need in a different SED for the BLR. The interpretation of He ii EW as an indicator of SED hardness can be directly tested by observations that cover the extreme-UV range (e.g.…”