Radiation Pressure Confinement - I. Ionized Gas in the ISM of AGN Hosts

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 7x10^4 L_{i,45} r_{50}^{-2} cm^{-3}, where L_{i,45} is the ionizing luminosity in units of 10^45 erg/s 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.Comment: Accepted for publication in MNRAS. The first author will be on regular email contact from December 201

What controls the [O III] 5007 line strength in AGN?

AGN display an extreme range in the narrow emission line equivalent widths. Specifically, in the PG quasar sample the equivalent width of the narrow [O III] 5007 line has a range of >300, while the broad Hb line, for example, has a range of 10 only. The strength of [O III] 5007 is modulated by the covering factor, CF, of the narrow line region (NLR) gas, its density n_e, and ionization parameter U. To explore which of these factors produces the observed large range in [O III] 5007 strength, we measure the strength of the matching narrow Hb and [O III] 4363 lines, detected in 40 out of the 87 z<0.5 PG quasars in the Boroson & Green sample. The photoionization code CLOUDY is then used to infer CF, n_e, and U in each object, assuming a single uniform emitting zone. We find that the range of CF (~0.02-0.2) contributes about twice as much as the range in both n_e and U towards modulating the strength of the [O III] 5007 line. The CF is inversely correlated with luminosity, but it is not correlated with L_Edd as previously speculated. The single zone [O III] 5007 emitting region is rather compact, having R=40L_44^0.45 pc. These emission lines can also be fit with an extreme two zone model, where [O III] 4363 is mostly emitted by a dense (n_e=10^7) inner zone at R=L_44^0.5 pc, and [O III] 5007 by a low density (n_e=10^3) extended outer zone at R=750L_44^0.34 pc. Such an extended [O III] 5007 emission should be well resolved by HST imaging of luminous AGN. Further constraints on the radial gas distribution in the NLR can be obtained from the spectral shape of the IR continuum emitted by the associated dust.Comment: A note was added in the proofs stag

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. IV. The Variable UV Absorption Lines

We report the detection of variable UV absorption lines in NGC 4395, based on UV observations with the HST STIS carried out in April and July, 2004, as part of a reverberation-mapping campaign. Low-ionization lines of O I, N I, Si II, C II, and Fe II, are present in the low-state spectra (April 2004) at a velocity v_shift=-250 km/s (system A_l), and additional high-ionization lines of C IV and N V appear in the high-state spectra (July 2004) at v_shift=-250 km/s (system A_h) and at v_shift=-840 km/s (system B). The absence of absorption from the low metastable levels of Si II implies a density <~10^3 cm^(-3) for system A_l, indicating a location outside the narrow line region (NLR). System A_h is peculiar as only N V absorption is clearly detected. A high N V/C IV absorption ratio is expected for a high metallicity absorber, but this is excluded here as the metallicity of the host galaxy and of the nuclear gas is significantly subsolar. A simple acceptable model for systems A_h and B is an absorber located between the broad line region (BLR) and the NLR, which absorbs only the continuum and the BLR. At the low-state the strong narrow emission lines of C IV and N V dominate the spectrum, making the absorption invisible. At the high-state the absorbed continuum and BLR emission dominate the spectrum. Thus, the change in the observed absorption does not reflect a change in the absorber, but rather a change in the continuum and BLR emission from behind the absorber, relative to the emission from the NLR in front of the absorber. Studies of the absorption line variability in highly variable objects can thus break the degeneracy in the absorber distance determination inherent to single epoch studies.Comment: Accepted for publication in The Astrophysical Journa