Radiation pressure confinement – I. Ionized gas in the ISM of AGN hosts

Abstract

We analyze the hydrostatic effect of AGN radiation pressure on optically thick gas in the host galaxy. We show that in luminous AGN, the radiation pressure likely confines the ionized layer of the illuminated gas. Radiation pressure confinement (RPC) has two main implications. First, the gas density near the ionization front is 7 × 10 4 L i,45 r −2 50 cm −3 , where L i,45 is the ionizing luminosity in units of 10 45 erg s −1 and r 50 is the distance of the gas from the nucleus in units of 50 pc. Second, as shown by Dopita et al., the solution of the ionization structure within each slab is unique, independent of the ambient pressure. We show that the RPC density vs. distance relation is observed over a dynamical range of ∼ 10 4 in distance, from sub-pc to kpc from the nucleus, and a range of ∼ 10 8 in gas density, from 10 3 to 10 11 cm −3 . This relation implies that the radiative force of luminous AGN can compress giant molecular clouds in the host galaxy, and possibly affect the star formation rate. The unique ionization structure in RPC includes a highly ionized X-ray emitting surface, an intermediate layer which emits coronal lines, and a lower ionization inner layer which emits optical lines. This structure can explain the observed overlap of the extended X-ray and optical narrow line emission in nearby AGN. We further support RPC by comparing the predicted ratios of the narrow lines strength and narrow line widths with available observations. We suggest a new method, based on the narrow line widths, to estimate the black hole mass of low luminosity AGN.

THE STACKED LYα EMISSION PROFILE FROM THE CIRCUM-GALACTIC MEDIUM OF z ∼ 2 QUASARS*

“…Following eqn. (10) in Stern et al 2014, using R = 275 kpc, the average ionizing photon flux for our sample Q ion ≡ 4πR 2 Φ LL ≈ 10 57 s −1 , L i,45 = Q ion h/ν = 6 × 10 46 erg s −1 , and τ dust = 0.05, we obtain n H = 0.7 × 10 −2 cm −3 , remarkably close to what we found. However, to establish a RPC scenario on such large scales one would need a particular tuning of the time scales as the photons need to reach 300 kpc from the source.…”

“…28 Intriguingly this value is in agreement with a radiation pressure confinement scenario (RPC; Stern et al 2014), shown to be in place at least up to 10 kpc from an AGN. In this scenario the radiation pressure is the main contributor to the gas pressure so that P rad ∼ Pgas, and the density profile is then shown to follow a simple relation.…”

EVIDENCE THAT MOST TYPE-1 AGNs ARE REDDENED BY DUST IN THE HOST ISM

“…This is not unexpected, since the Balmer photons are likely extincted by dust within the NLR clouds, thus increasing (Hα/Hβ) int above the Case B value (see fig. 6 in Stern, Laor, & Baskin 2014, and also Dopita et al 2002). The observed slope of the relation between (Hα/Hβ) obs and α opt in the NLR is shallower than the expected slope derived in eqn.…”

“…We include in the calculation the effect of radiation pressure on the dust grains, which is significant at U 0.01 (Dopita et al 2002;Groves, Dopita, & Sutherland 2004;Stern et al 2014). We use the depleted ISM abundance set implemented in cloudy, except for the sodium abundance which we assume has solar abundance.…”

A Universal Density Structure for Circumgalactic Gas

“…RPC conditions have been shown to apply in at least some star forming regions (Draine 2011;Yeh & Matzner 2012;Yeh et al 2013;Verdolini et al 2013), in AGN emission line regions (Dopita et al 2002;Baskin et al 2014a;Stern et al 2014aStern et al , 2016, in AGN absorption line regions Baskin et al 2014b), and possibly in the CGM of quasar hosts, in the part exposed to the quasar radiation (Arrigoni Battaia et al 2016). Assuming that dust grains are embedded in the CGM gas, as suggested by the results of Ménard et al (2010), then the dominant contribution to the integral in eqn.…”