What the Optical Recombination Lines Can Tell Us about the Broad-Line Regions of Active Galactic Nuclei

By K. Korista, M. Goad. 2004

192 citations recorded by the source corpus.

We investigate the effect of a global change in the ionizing continuum level on the behavior of the strong optical broad emission lines seen in spectra of the nuclear emission-line regions of active galactic nuclei (AGNs), including the Balmer lines, He I λ5876, and He II λ4686. Unlike most of the prominent heavy-element lines found in the UV, the optical hydrogen and helium recombination lines' emissivities are strongly dependent on the incident continuum flux, since these lines arise out of excited states whose optical depths depend on the incident flux of photons. Using photoionization calculations, we determine the luminosity-dependent responsivities, η(r, L(t)) = Δ log Lline/Δ log Lcont, of these lines for a general model of the broad emission line region (BLR), with the purpose of establishing them as important probes of the physical conditions within the BLRs of AGNs. The dependence of these lines' emissivities on the incident photon flux invokes a dependence in their responsivities on distance from the central continuum source. In particular, the responsivities of these lines are generally anticorrelated with the incident photon flux. Thus, their responsivities vary with distance within the BLR for a fixed continuum luminosity and change with time as the continuum source varies. Consequently, after correcting for light-travel-time effects, the response of the Balmer and optical helium lines should generally be strongest during low continuum luminosity states. Responsivity that depends on photon flux and continuum state may explain a number of outstanding problems currently under investigation in broad-line variability studies of these and other emission lines. These include the origin of the intrinsic Baldwin effect, measurements of luminosity-dependent lags (a "breathing" BLR), and luminosity-dependent variations in integrated broad emission line flux ratios (including He II λ4686/Hβ), broad-line profile shapes, and radial velocity-dependent intensity ratios. The broad Hα/Hβ and He I/Hβ flux ratios and the Balmer emission line responsivity are observed to decrease from the line center to the line wings. These, along with our findings, lead to the conclusion that the BLR velocity field diminishes with increasing distance from the central continuum source. This is consistent with recent reverberation studies that find a relationship between the emission-line lag and rms profile width for multiple lines in individual AGNs, which implies that the velocity field is dominated by a central massive object. Finally, the responsivity of ionization-bounded clouds can account for much of the observed behavior of the optical recombination lines (e.g., the weak response of the Balmer line wings) previously attributed to a substantial contribution from matter-bounded clouds at small BLR radii.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/af81af9dff13a3fe426e4f35c600ac935ea0c0d4

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Instrumentation

Citations in the corpus

PS1-10jh: THE DISRUPTION OF A MAIN-SEQUENCE STAR OF NEAR-SOLAR COMPOSITION (2013, methodology): “If the dissipation mechanism within the debris disks resulting from tidal disruptions is similar to the dissipation mechanism that controls angular momentum transport in steadily-accreting AGN, it is reasonable to expect that the two structures should have many similarities in terms of their density”
AN ALTERNATIVE APPROACH TO MEASURING REVERBERATION LAGS IN ACTIVE GALACTIC NUCLEI (2010, result): “Using almost the same set of light curves from NGC 5548 (we add year 14 and exclude year 13), Cackett & Horne (2006) find a much shallower slope (0.1–0.46) with a luminosity-dependent delay map, in better agreement with the prediction of photoionization models (∼ 0.23; Korista & Goad 2004).”
The Sloan Digital Sky Survey Reverberation Mapping Project: Hα and Hβ Reverberation Measurements from First-year Spectroscopy and Photometry (2017, background): “…due to photoionization predictions, with radial stratification and optical-depth effects causing the Hα emission line to appear at larger distances than Hβ (Netzer 1975; Rees et al. 1989; Korista & Goad 2004); see Section 4.3 of Bentz et al. (2010) for a more detailed discussion of this phenomenon.”
THE LICK AGN MONITORING PROJECT 2011: SPECTROSCOPIC CAMPAIGN AND EMISSION-LINE LIGHT CURVES (2015, background): “…and luminosity arises as a result of the relationship between the ionizing flux and the local responsivity (or reprocessing efficiency) of BLR gas: the Hβ responsivity is highest in the outer portions of the BLR where incident continuum fluxes are lower (Korista & Goad 2004; Goad & Korista 2014).”
The Effect of Radiation Pressure on Virial Black Hole Mass Estimates and the Case of Narrow-Line Seyfert 1 Galaxies (2008, background): “…framework of the standard BLR model, photoionization calculations can explain observed spectra only if BLR clouds are optically thick to ionizing radiation and adoptedNH are usually of the order of 1023cm−2 (e.g. Baldwin et al. 1995; Kaspi & Netzer 1999; Korista & Goad 2004 and references therein).”
THE LICK AGN MONITORING PROJECT: REVERBERATION MAPPING OF OPTICAL HYDROGEN AND HELIUM RECOMBINATION LINES (2010, methodology): “Here, we focus on the specific predictions for the optical recombination lines presented by Korista & Goad (2004).”
Broad-line Balmer decrements in blue active galactic nuclei (2007, methodology): “…by BLR photoionization modeling (e.g., Netzer 1975, Kwan & Krolik 1981, Canfield & Puetter 1981, Collin-Souffrin et al. 1982, Rees et al. 1989, Dumont et al. 1998, Korista & Goad 2004, see Baldwin 1997 for a review), the broad-line Hα/Hβ ratio (Hαb/Hβb) varies widely in different BLR conditions.”
REVERBERATION MAPPING MEASUREMENTS OF BLACK HOLE MASSES IN SIX LOCAL SEYFERT GALAXIES (2010, background): “…the rms line profiles of these objects were clearly narrower than their mean profiles (the rms profile is often narrower than the mean profile, which is not surprising, given that likely not all flux seen in the mean spectrum varies in response to the continuum; see, e.g., Korista & Goad 2004).”
Evolution of broad-line emission from active galactic nuclei (2013, unknown): “This systematic behavior is at odds with the fine-tuning required by explanations of the intermediate types that were based on partial obscuration or transient radiative transfer effects (e.g., Korista & Goad 2004).”
Discovery of extreme [O III] λ5007 Å outflows in high-redshift red quasars (2015, background): “One possible explanation is that in fact the true Hα/Hβ ratios are smaller than Case B, which is difficult (Gaskell & Ferland 1984), but not impossible (Korista & Goad 2004).”