This review describes recent developments related to the unified model of active galactic nuclei (AGNs). It focuses on new ideas about the origin and properties of the central obscurer (torus) and ...the connection to its surroundings. The review does not address radio unification. AGN tori must be clumpy but uncertainties about their properties persist. Today's most promising models involve disk winds of various types and hydrodynamic simulations that link the large-scale galactic disk to the inner accretion flow. Infrared (IR) studies greatly improved our understanding of the spectral energy distribution of AGNs, but they are hindered by various selection effects. X-ray samples are more complete. The dependence of the covering factor of the torus on luminosity is a basic relationship that remains unexplained. There is also much confusion regarding real type-II AGNs, which do not fit into a simple unification scheme. The most impressive recent results are due to IR interferometry, which is not in accord with most torus models, and the accurate mapping of central ionization cones. AGN unification may not apply to merging systems and is possibly restricted to secularly evolving galaxies.
ABSTRACT
The bolometric luminosity of active galactic nuclei (AGNs) is difficult to determine, and various approximations have been used to calibrate it against different observed properties. Here, I ...combine theoretical calculations of optically thick, geometrically thin accretion discs, and observed X-ray properties of AGN, to provide new bolometric correction factors (kBOL) over a large range of black hole (BH) mass, accretion rate, and spin. This is particularly important in cases where the mass accretion rate cannot be determined from the observed spectral energy distribution, and in cases where luminosity-independent correction factors have been used. Simple power-law approximations of kBOL are provided for L(5100 Å), L(3000 Å), L(1400 Å), L(2–10 keV), and L(narrow Hβ). In all cases, the uncertainties are large mostly due to the unknown BH spin. Prior knowledge of the BH mass reduces the uncertainty considerably.
Accretion and star formation (SF) rates in low-redshift Sloan Digital Sky Survey type II active galactic nuclei (AGN) are critically evaluated. Comparison with photoionization models indicates that ...bolometric luminosity (Lbol) estimates based on L(O iii λ5007) severely underestimate Lbol in low-ionization sources such as LINERs. An alternative method based on L(Hβ) is less sensitive to the ionization level, and a novel method, based on a combination of L(O iii λ5007) and L(O i λ6300), is perhaps the best. Using this method, I show that low-ionization AGN are accreting faster than assumed until now. Other significant related findings are as follows. (1) Any type II AGN property related to the black hole (BH) mass is more reliably obtained by removing blue galaxies from the sample. (2) Seyfert 2s and LINER 2s form a continuous sequence of L/LEdd with no indication for a change in accretion mechanism or mode of mass supply. There are very few, if any, LINERs in all type I samples which results in a much narrower L/LEdd distribution compared with type II samples. (3) There is a strong correlation between the SF luminosity, LSF, and Lbol over more than five orders of magnitude in luminosity. This leads to a simple relationship between bulge and BH growth rates, g(bulge)/g(BH) ∝L−0.2bol, where g(bulge)/g(BH) ≃ 115 for Lbol= 1042 erg s−1. Seyfert 2s and LINER 2s follow the same LSF–Lbol correlation for all sources with a stellar age indicator, Dn4000, smaller than 1.8. This suggests that a similar fraction of SF gas finds its way to the centre in all AGN. (4) Lbol, LSF, L/LEdd and the specific SF rate follow Dn4000 in a similar way.
Research into active galactic nuclei (AGN) – the compact, luminous hearts of many galaxies – is at the forefront of modern astrophysics. Understanding these objects requires extensive knowledge in ...many different areas: accretion disks, the physics of dust and ionized gas, astronomical spectroscopy, star formation, and the cosmological evolution of galaxies and black holes. This new text by Hagai Netzer, a renowned astronomer and leader in the field, provides a comprehensive introduction to the theory underpinning our study of AGN and the ways that we observe them. It emphasizes the basic physics underlying AGN, the different types of active galaxies and their various components, and the complex interplay between them and other astronomical objects. Recent developments regarding the evolutionary connections between active galaxies and star-forming galaxies are explained in detail. Both graduate students and researchers will benefit from Netzer's authoritative contributions to this exciting field of research.
The distributions of L(O III l5007), black hole (BH) mass and L/L Edd in two large samples of type-I and type-II active galactic nuclei (AGNs) are compared in order to test the suggestion that ...radiation pressure force is affecting the gas velocity in the broad line region and hence the BH mass determination. The samples are drawn from the Sloan Digital Sky Survey archive and are modified to represent the same parent distribution at 0.1 <= z <= 0.2. BH masses in type-I sources are calculated in two different ways, one using a simple virial mass assumption and the other by taking into account the effect of radiation pressure force on the gas. The simple virial-mass estimate results in good agreement with the s*-based BH mass and L/L Edd estimates in type-II sources. In contrast, there is a clear disagreement in the L/L Edd distributions when radiation pressure-based estimates are used. This indicates that radiation pressure force is not important in 0.1 <= z <= 0.2 AGNs with L 5100=1042.8-44.8 erg s-1. This has important implications to the physics of the gas producing the broad emission lines in AGNs, in particular the existence of extremely large column density (~1024 cm-2) clouds.
We present a detailed investigation of the near-infrared to far-infrared (near-IR to far-IR) spectral energy distribution (SED) of a large sample of Spitzer-observed active galactic nuclei (AGNs). We ...fitted the spectra of 51 narrow-line Seyfert 1 galaxies (NLS1s) and 64 broad-line Seyfert 1 galaxies (BLS1s) using a three-component model: a clumpy torus, a dusty narrow-line region and hot pure-graphite dust located in the outer part of the broad-line region (BLR). The fitting was performed on star formation (SF) subtracted SEDs using SF templates that take into account the entire range of possible host galaxy properties. We find that the mid-IR intrinsic emission of NLS1s and BLS1s is very similar, regardless of the AGN luminosity, with long-wavelength downturn at around 20-25 μm. We present a detailed model of the hot-dust component that takes into account the distribution of dust temperature within the clouds and their emission-line spectrum. The hot-dust continuum provides a very good fit to the observed near-IR continuum spectrum. Most line emission in this component is dramatically suppressed, except Mg ii λ2798 and He i lines that are still contributing significantly to the total BLR spectrum. We calculate the covering factors (CFs) of all the AGN components and show that the CF of the hot-dust clouds is about 0.15-0.35, similar to the CF of the torus, and is anticorrelated with the source luminosity and the normalized accretion rate.
We present new ALMA band-7 data for a sample of six luminous quasars at , powered by fast-growing supermassive black holes (SMBHs) with rather uniform properties: the typical accretion rates and ...black hole masses are and . Our sample consists of three "FIR-bright" sources, which were individually detected in previous Herschel/SPIRE observations, with star formation rates of , and three "FIR-faint" sources for which Herschel stacking analysis implies a typical SFR of ∼400 . The dusty interstellar medium in the hosts of all six quasars is clearly detected in the ALMA data and resolved on scales of ∼2 kpc, in both continuum ( ) and line emission. The continuum emission is in good agreement with the expectations from the Herschel data, confirming the intense SF activity in the quasar hosts. Importantly, we detect companion sub-millimeter galaxies (SMGs) for three sources-one FIR-bright and two FIR-faint, separated by and from the quasar hosts. The -based dynamical mass estimates for the interacting SMGs are within a factor of ∼3 of the quasar hosts' masses, while the continuum emission implies . Our ALMA data therefore clearly support the idea that major mergers are important drivers for rapid early SMBH growth. However, the fact that not all high-SFR quasar hosts are accompanied by interacting SMGs and the gas kinematics as observed by ALMA suggest that other processes may be fueling these systems. Our analysis thus demonstrates the diversity of host galaxy properties and gas accretion mechanisms associated with early and rapid SMBH growth.
Abstract
We present a new analysis of the Palomar–Green quasar sample based on
$\it Spitzer$
and
$\it Herschel$
observations. (i) Assuming polycyclic aromatic hydrocarbon (PAH)-based star formation ...luminosities (L
SF) similar to Symeonidis et al. (S16), we find mean and median intrinsic active galactic nucleus (AGN) spectral energy distributions (SEDs). These, in the far-infrared (FIR), appear hotter and significantly less luminous than the S16 mean intrinsic AGN SED. The differences are mostly due to our normalization of the individual SED that properly accounts for a small number of very FIR-luminous quasars. Our median, PAH-based SED represents ∼6 per cent increase on the 1–243 μm luminosity of the extended Mor & Netzer (EM12) torus SED, while S16 find a significantly larger difference. It requires large-scale dust with T ∼ 20-30 K, which, if optically thin and heated by the AGN, would be outside the host galaxy. (ii) We also explore the black hole and stellar mass growths, using L
SF estimates from fitting
$\it Herschel$
/PACS observations after subtracting the EM12 torus contribution. We use rough estimates of stellar mass, based on scaling relations, to divide our sample into groups: on, below and above the star formation main sequence (SFMS). Objects on the SFMS show a strong correlation between star formation luminosity and AGN bolometric luminosity, with a logarithmic slope of ∼0.7. Finally, we derive the relative duty cycles of this and another sample of very luminous AGN at z = 2–3.5. Large differences in this quantity indicate different evolutionary pathways for these two populations characterized by significantly different black hole masses.
ABSTRACT
High cadence, high quality observations of active galactic nuclei (AGN) clearly show continuum variations with lags, relative to the shortest observed variable UV continuum that increase ...with wavelength (‘lag spectra’). These have been attributed to the irradiation and heating of the central accretion disc by the central X-ray emitting corona. An alternative explanation, connecting the observed lag-spectra to line and continuum emission from gas in the broad line region (BLR), has also been proposed. In this paper I show the clear spectral signature of the time-dependent diffuse gas emission in the lag-spectrum of 6 AGN. I also show a new lag–luminosity relationship for 9 objects which is a scaled down version of the well known τ(Hβ)–L5100 relationship in AGN. The shape of the lag-spectrum, and its normalization, are entirely consistent with diffuse emission from radiation pressure supported clouds in a BLR with a covering factor of about 0.2. While some contributions to the continuum lag from the irradiated disc cannot be excluded, there is no need for this explanation.
We fitted Spitzer/IRS ~ 2-35 Delta *mm spectra of 26 luminous quasi-stellar objects in an attempt to define the main emission components. Our model has three major components: a clumpy torus, dusty ...narrow-line region (NLR) clouds, and a blackbody-like dust. The models utilize the clumpy torus of Nenkova et al. and are the first to allow its consistent check in type-I active galactic nuclei (AGNs). Single torus models and combined torus-NLR models fail to fit the spectra of most sources, but three-component models adequately fit the spectra of all sources. We present torus inclination, cloud distribution, covering factor, and torus mass for all sources and compare them with bolometric luminosity, black hole mass, and accretion rate. The torus mass is found to be correlated with the bolometric luminosity of the sources. Torus-covering factor may also be (anti-)correlated, if some possibly anomalous points are omitted. We find that a substantial amount of the ~2-7 Delta *mm radiation originates from a hot dust component, which is likely situated in the innermost part of the torus. The luminosity radiated by this component and its covering factor are comparable to those of the torus. We quantify the emission by the NLR clouds and estimate their distance from the center. The distances are ~700 times larger than the dust sublimation radius, and the NLR-covering factor is about 0.07. The total covering factor by all components is in good agreement with the known AGN type-I:type-II ratio.
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