The dust origin of the Broad Line Region and the model consequences for AGN unification scheme☆
Introduction
Broad emission lines seen in the optical and UV spectra are the most characteristic features of active galaxies. They led to the discovery of quasars (Schmidt, 1963), and they remain one of the most powerful tools to study the accretion processes onto supermassive black holes, the outflows from the nuclear region, and the role of Active Galactic Nuclei (AGN) in shaping the host galaxy. Since the Broad Line Region (BLR) is very compact it remains unresolved. Our knowledge of this region relies on the spectral analysis, including the spectra variability, and on the theoretical interpretation of the processes responsible for the observed emission of radiation.
BLR properties have been studied in a numerous papers and books (for general reviews, see Krolik, 1999, Netzer, 2013). The structure of the region is complex, since part of the radiation comes likely from the surface of an accretion disk surrounding the central black hole and the other part comes from the gas well above the disk. The dynamics is equally complex, and the reverberation studies show both the Keplerian component to the cloud motion, further complicated by the possible spiral structures in the disk as well as strong turbulent motion, inflow and outflow (e.g. Shapovalova et al., 4151, Grier et al., 2013).
It was postulated long time ago that the broad emission lines should actually be divided into two distinct components. Low Ionization Lines (LIL), like H, Mg II, or Fe II should form in higher density region, likely closer to the disk or within the disk. High Ionization Lines (HIL) are emitted by the lower density regions (Collin-Souffrin et al., 1988). The dynamics of the two regions seems also different: LIL does not show a strong inflow/outflow while an outflow is characteristic for HIL. The location of HIL is also close to the central black hole, as indicated by the time delays between the lines and the continuum.
In this paper we further concentrate on the LIL part. Their characteristic properties can be summarized in the following way: (i) line profiles is usually inconsistent with their origin in a Keplerian disk; in Narrow Line Seyfert 1 galaxies and type A quasars they are well represented by a simple Lorentzian symmetric profile; double-peak profile is seen only in a small fraction of objects (e.g. Eracleous et al., 2009) (ii) the lines do not indicate strong net inflow/outflow (iii) the covering factor is high; about 30% of the nuclear emission is intercepted by the corresponding part of the BLR (iv) the size of this region scales well with the square root of the monochromatic flux of an active galaxy, as showed in numerous reverberation studies (e.g. Kaspi et al., 2000, Peterson et al., 2004, Bentz et al., 2009).
This means that the material cannot be entirely close to the disk, since in that case the covering factor would be small. It cannot also be a massive wind or an inflow, since then the lines would be Doppler-shifted with respect to the host galaxy. However, the material has to have an opportunity to appear high above the disk to intercept enough of the central radiation flux.
So far three mechanisms of the formation of the BLR has been proposed in the past: (i) magnetic wind (Elitzur and Shlosman, 2006, Elitzur and Ho, 2009, Elitzur et al., 2014), (ii) accretion disk instability due to self-gravity (Collin and Zahn, 1999; see also Collin and Zahn, 2008, Wang et al., 2012) (iii) the outflow connected with the transition between the radiation-pressure dominance and the gas-pressure dominance in the accretion disk (Risaliti and Elvis, 2010).
Here we consider the fourth model: the Failed Radiatively Accelerated Dust-driven Outflow (hereafter FRADO) which well explains the above properties, and we analyze the predictions of this model which can be used to prove or falsify the proposed mechanism.
Section snippets
Failed Radiatively Accelerated Dust-driven Outflow (FRADO) model of the BLR
The model has been proposed by Czerny and Hryniewicz (2011) and tested against the data for the most studied Seyfert 1 galaxy NGC 5548 by Galianni and Horne (2013). Here we summarize the basic outline of the model.
The presence of dust was extensively discussed in the context of the dusty-molecular torus, which was introduced by Antonucci and Miller (1985) in order to explain the spectra of Seyfert 2 galaxies. The torus is located much further from the nucleus than the BLR. At the distance of
Tests of the FRADO model
Since there are other models of the formation of the BLR regions, each of them should be tested against observations. Here we treat the theory as a starting point, and formulate specific predictions which results from the assumptions underlying the model.
Discussion
The Failed Radiatively Accelerated Dust-driven Outflow (FRADO) model is an attractive new way to explain the basic mechanism of the formation of the BLR in active galaxies. It is very simple and explains the basic observational facts, like the scaling of the size of the BLR with the square root of the monochromatic luminosity, the presence of the material well above the disk, and the absence of strong outflow signatures in the lines like H and Mg II.
However, the model is still in its infancy
Acknowledgements
The spectroscopic observations reported in this paper were obtained with the Southern African Large Telescope (SALT), proposals 2012-1-POL-008. JM, BC, FP, MK, and AŚ acknowledge the support by the Foundation for Polish Science through the Master/Mistrz program 3/2012. K.H. also thanks the Scientific Exchange Programme (Sciex) NMSch for the opportunity of working at ISDC. Part of this work was supported by Polish Grants Nr. 719/NSALT/2010/0, UMO-2012/07/B/ST9/04425 and by Polish National
References (80)
- et al.
Double-peaked emission lines as a probe of the broad-line regions of active galactic nuclei
NewAR
(2009) - et al.
Reverberation time lags in the high luminosity quasar PG 1247+267
AdSpR
(2014) - et al.
Star formation in self-gravitating disks in active galactic nuclei. II. Episodic formation of broad-line regions
ApJ
(2012) - et al.
Self-shadowing effects of slim accretion disks in active galactic nuclei: diverse appearance of the broad-line region
ApJ
(2014) - et al.
Slim accretion disks
ApJ
(1988) - et al.
Spectropolarimetry and the nature of NGC 1068
ApJ
(1985) - et al.
The HST view of the broad line region in low luminosity AGN
A&A
(2014) - et al.
The mass of quasar broad emission line regions
ApJ
(2003) - et al.
The lick AGN monitoring project 2011: Fe II reverberation from the outer broad-line region
ApJ
(2013) - et al.
The radius–luminosity relationship for active galactic nuclei: the effect of host-galaxy starlight on luminosity measurements. II. The full sample of reverberation-mapped AGNs
ApJ
(2009)
Modeling Fe II emission and revised Fe II (UV) empirical templates for the Seyfert 1 galaxy I Zw 1
ApJ
Warping and precession in galactic and extragalactic accretion disks
ApJ
On the performance of quasar reverberation mapping in the era of time-domain photometric surveys
ApJ
The environment of active galactic nuclei I – a two-component broad emission line model
MNRAS
Star formation and evolution in accretion disks around massive black holes
A&A
Star formation in accretion discs: from the Galactic center to active galactic nuclei
A&A
Constraints on quasar accretion disks from the optical/ultraviolet/soft X-ray big bump
ApJ
Constraints on the black hole spin in the quasar SDSS J094533.99+100950.1
MNRAS
The origin of the broad line region in active galactic nuclei
A&A
Universal spectral shape of high accretion rate AGN
A&A
Constraints for the accretion disk evaporation rate in AGN from the existence of the broad line region
A&A
The UV continuum of quasars: models and SDSS spectral slopes
ApJ
Broad-line Balmer decrements in blue active galactic nuclei
MNRAS
Supermassive black holes with high accretion rates in active galactic nuclei. I. First results from a new reverberation mapping campaign
ApJ
On the disappearance of the broad-line region in low-luminosity AGN
ApJ
Evolution of broad-line emission from active galactic nuclei
MNRAS
The AGN obscuring torus – End of the doughnut paradigm?
ApJ
Implications of infalling Fe II-emitting clouds in active galactic nuclei: anisotropic properties
ApJ
A test of the failed disc wind scenario for the origin of the broad-line region in active galactic nuclei
MNRAS
Stratified quasar winds: integrating X-ray and infrared views of broad absorption line quasars
ASPC
What broad emission lines tell us about how active galactic nuclei work
NewAR
Line shifts, broad-line region inflow, and the feeding of active galactic nuclei
ApJ
Off-axis variability of AGNs: a new paradigm for broad lines and continuum emitting regions
Balt. A
The structure of the broad-line region in active galactic nuclei. I. Reconstructed velocity-delay maps
ApJ
The chemical evolution of QSOs and the implications for cosmology and galaxy formation
ApJ
The warp in the subparsec molecular disk in NGC4258 as an Explanation for persistent asymmetries in the maser spectrum
ApJ
SDSS J094533.99+100950.1– the remarkable weak emission line quasar
MNRAS
SALT long-slit spectroscopy of LBQS 2113–4538: variability of the Mg II and Fe II component
A&A
On the dynamics of a twisted disc immersed in a radiation field
MNRAS
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Based on the talk presented during the COSPAR 2014 meeting; based on observations made with the Southern African Large Telescope (SALT) under Program 2012-1-POL-008 (PI: B. Czerny).