The case for inflow of the broad-line region of active galactic nuclei

Abstract

The high-ionization lines of the broad-line region (BLR) of thermal active galactic nuclei (AGNs) show blueshifts of a few hundred km/s to several thousand km/sec with respect to the low-ionization lines. This has long been thought to be due to the highionization lines of the BLR arising in a wind of which the far side of the outflow is blocked from our view by the accretion disc. Evidence for and against the discwind model is discussed. The biggest problem for the model is that velocity-resolved reverberation mapping repeatedly fails to show the expected kinematic signature of outflow of the BLR. The disc-wind model also cannot readily reproduce the red side of the line profiles of high-ionization lines. The rapidly falling density in an outflow makes it difficult to obtain high equivalent widths. We point out a number of major problems with associating the BLR with the outflows producing broad absorption lines. An explanation which avoids all these problems and satisfies the constraints of both the line profiles and velocity-resolved reverberation-mapping is a model in which the blueshifting is due to scattering off material spiraling inwards with an inflow velocity of half the velocity of the blueshifting. We discuss how recent reverberation mapping results are consistent with the scattering-plus-inflow model but do not support a disc-wind model. We propose that the anti-correlation of the apparent redshifting of Hβ with the blueshifting of C IV is a consequence of contamination of the red wings of Hβ by the broad wings of [O III].

A Test of the Formation Mechanism of the Broad Line Region in Active Galactic Nuclei

“…Czerny et al 2004, Balmaverde & Capetti 2014, and the references therein), the significant change in the structure of the underlying disk should affect somehow the BLR. However, if the BLR lines form directly in the inflowing material (see Gaskell & Goosmann 2016) or come from the material temporarily ejected from the disk by passing stars (Zurek, Siemiginowska & Colgate 1994) then the physical state of the underlying disk becomes relatively unimportant. However, in this case we do not have any explanation for the observed correlation between the BLR size and the monochromatic flux.…”

Space Telescope and Optical Reverberation Mapping Project. V. Optical Spectroscopic Campaign and Emission-line Analysis for NGC 5548

“…Gas with very low v r could have a wide range of lags, and a spherical or flat disk distribution of BLR clouds in Keplerian motion could lead to a double-peaked velocityresolved lag profile if the ionizing source is emitting anisotropically Goad & Wanders 1996;Horne et al 2004). Previous studies of the UV and optical lines in NGC 5548 have inferred either Keplerian orbits Wanders et al 1995;Denney et al 2009;Bentz et al 2010b) or infalling motion (Crenshaw & Blackwell 1990;Done & Krolik 1996;Welsh et al 2007;Pancoast et al 2014;Gaskell & Goosmann 2016) for the BLR gas. From our data, the shape of the Hβ velocity-resolved lag profile suggests a BLR dominated by Keplerian motion.…”

“…The geometry and kinematics of the BLR play a significant role in AGN research because these properties can be used to infer the mass of the central black hole (BH; e.g., Gaskell & Sparke 1986;Clavel et al 1991;Kaspi et al 2000;Denney et al 2006Denney et al , 2010Pancoast et al 2014). Additionally, it is possible that infalling BLR gas may fuel SMBH accretion (e.g., Peterson 2006;Gaskell & Goosmann 2016) and outflowing gas may be part of disk winds that carry away angular momentum from the disk and provide energy and momentum feedback to the host galaxy (e.g., Emmering et al 1992;Murray & Chiang 1997;Kollatschny 2003;Leighly & Moore 2004). Understanding the dynamical state and physical conditions of gas in the BLR is of key importance in completing our understanding of the AGN phenomenon.…”

Using the Properties of Broad Absorption Line Quasars to Illuminate Quasar Structure

“…In the simplest outflowing disk-wind model, it is expected that there will be little lag in the blue line wing compared to the variation in the continuum, while the red wing experiences up to twice the delay (Gaskell 2009). Gaskell & Goosmann (2016) suggested that the shorter response in the red wing is an evidence of inflow motion, and hence disfavouring the outflowing wind model. However, Mangham et al (2017) using detailed radiative transfer and ionisation treatment of a disk-wind simulation argued that the classical indicator of Keplerian rotation, inflow, or outflow is not always denoted by the symmetric, red wing leading blue wing, or blue wing leading red wing line profile signature.…”