We argue that supermassive black hole growth in AGN occurs via sequences of randomly--oriented accretion discs with angular momentum limited by self--gravity. These stably co-- or counter--align with the black hole spin with almost equal frequency. Accretion from these discs very rapidly adjusts the hole's spin parameter to average values $\bar a \sim 0.1-0.3$ (the precise range depending slightly on the disc vertical viscosity coefficient $α_2$) from any initial conditions, but with significant fluctuations ($Δa\sim \pm 0.2$) about these. We conclude (a) AGN black holes should on average spin moderately, with the mean value $\bar a$ decreasing slowly as the mass increases; (b) SMBH coalescences leave little long--term effect on $\bar a$; (c) SMBH coalescence products in general have modest recoil velocities, so that there is little likelihood of their being ejected from the host galaxy; (d) black holes can grow even from stellar masses to $\sim 5\times 10^9 \msun$ at high redshift $z\sim 6$; (e) jets produced in successive accretion episodes can have similar directions, but after several episodes the jet direction deviates significantly. Rare examples of massive holes with larger spin parameters could result from prograde coalescences with SMBH of similar mass, and are most likely to be found in giant ellipticals. We compare these results with observation.
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Using the gaseous disk model and the condition for emission based on a stimulated origin of thermal radio emission (Prigara 2003), we received the wavelength dependence of polarization for radio emission from active galactic nuclei consistent with available observational data. The strength of magnetic field required to produce an observable degree of polarization in active galactic nuclei is discussed.
This article gives a brief overview of the direct and indirect evidence in support of the standard paradigm that active galactic nuclei are powered by supermassive black holes.
Hard X-ray emission is ubiquitous in accreting black holes, both in Galactic binary systems and in Active Galactic Nuclei. I review the different spectra which can be seen from these systems, and possible ways of producing this emission. X-ray reflection should give an observational test of these scenarios, but we need better models before this can give an unambiguous diagnostic of the accretion geometry.
It has long been suspected that Active Galactic Nuclei are powered by accretion of matter onto massive black holes and this belief implies their presence in the nuclei of most nearby galaxies as "relics" of past activity. Just a few years ago this was considered a paradigm but, recently, new ground-based and Hubble Space Telescope observations are producing a breakthrough in our knowledge on massive black holes. I will review the evidence for the existence of black holes in galactic nuclei and how their presence is related to host galaxy properties and AGN activity.
The pointing directions of extensive air showers observed at the Pierre Auger Observatory were fitted within 3.1 degree with positions of the nearby active galactic nuclei from the Veron-Cetty and P. Veron catalog. The cosmic ray luminosity of the active galactic nuclei which happened to be a source of the particular cosmic ray event constitutes a fraction ~0.0001 of the optical one if only cosmic ray particles with energies above 60 EeV are produced. If produced cosmic ray particles have a spectrum dE/E^3 up to ~100 GeV then the cosmic ray luminosity would be much higher than the optical one …
Many nearby galaxies contain optical signatures of nuclear activity in the form of LINER nuclei. LINERs may be the weakest and most common manifestation of the quasar phenomenon. The physical origin of this class of objects, however, has been ambiguous. I draw upon a number of recent observations to argue that a significant fraction of LINERs are low-luminosity active galactic nuclei.
Dust plays an essential role in the unification theory of active galactic nuclei (AGNs). This review summarizes our current understanding of the extinction and infrared emission properties of the circumnuclear dust in AGNs as well as the inferred dust composition and size distribution.
We study in detail the properties of the accreting corona model for active galactic nuclei and galactic black holes. In this model the fraction of the energy liberated in the corona at a given radius is calculated from the global parameters of the model (mass of the central object, accretion rate and viscosity parameter) and it appears to be a strong function of the radius. The model predicts the relative decrease of the coronal hard X-ray emission with an increase of the accretion rate. The presented description of disc/corona interaction forms a basis for further studies of disc disruption m…
Observations give strong support for the unification scheme of active galactic nuclei. The scheme is premised on toroidal obscuration of the central engine by dusty clouds that are individually very optically thick. These lectures summarize the torus properties, describe the handling and implications of its clumpy nature and present speculations about its dynamic origin.
Masses of black holes in nearby galactic nuclei can be measured in a variety of ways, using stellar and gaseous kinematics. Reliable black hole masses are known for several dozen objects, so that demographic questions can start to be addressed with some confidence. Prospects for the near future are discussed briefly.
The dynamical evidence for black holes (BHs) in galactic nuclei is reviewed, with emphasis on recent improvements in spatial resolution, methods for analyzing galaxy spectra and dynamical modeling. M31, M32 and M87 are discussed in some detail.
Galactic nuclei often harbor a disproportionately large amount of star formation activity with respect to their surrounding disks. Not coincidentally, the density of molecular material in galactic nuclei is often also much greater than that in disks (Table 1 in Kennicutt 1998). The interplay between rich populations of young stars and dense molecular environments is evident in our own Galactic center, which hosts over 10% of Galactic star formation activity within only $<$0.1% of the volume of the Galactic disk. Data obtained with the VLA and HST reveal a variety of star forming sites in the G…
I review the status of observational determinations of central masses in nearby galactic nuclei. Results from a variety of techniques are summarized, including ground-based and space-based optical spectroscopy, radio VLBI measurements of luminous water vapor masers, and variability monitoring studies of active galactic nuclei. I will also discuss recent X-ray observations that indicate relativistic motions arising from the accretion disks of active nuclei. The existing evidence suggests that supermassive black holes are an integral component of galactic structure, at least in elliptical and bu…
Recent works, both theoretical and observational, have suggested that turbulence could play a non-negligible role in the broadening of emission lines in active galactic nuclei. The purpose of this note is to show how shock wave-turbulence interaction, under unsteady regime, can affect the broadening of emission lines.
We discuss how several of the questions that remain unclear on the physics of Active Galactic Nuclei may find elements of answers when using in the coming years the extraordinary set of instruments that will be flying simultaneously to observe in all bands of the electromagnetic spectrum. The choice of questions mentioned here is personal and not exhaustive.
The masses of the central black holes of Active Galactic Nuclei estimated by the reverberation virial technique seem to be significantly smaller than predicted from the black hole mass-bulge luminosity relation for massive black holes detected by kinematic methods in less active and normal galaxies. We reexamine the relation for updated data and suggest a few reasons that may cause this discrepancy.
The analysis of eight EXOSAT X-ray lightcurves of six active galactic nuclei by nonlinear prediction methods indicates that the observed short time-scale variability is truly stochastic and is not caused by deterministic chaos. This result favors X-ray emission models with mutliple centers of production of hot, possibly relativistic electrons.
Reverberation mapping is a proven technique that is used to measure the size of the broad emission-line region and central black hole mass in active galactic nuclei. More ambitious reverberation mapping programs that are well within the capabilities of Hubble Space Telescope could allow us to determine the nature and flow of line-emitting gas in active nuclei and to assess accurately the systematic uncertainties in reverberation-based black hole mass measurements.
Motivated by the finding that the observed surface brightness profile of many galactic nuclei are well fit by double-power-laws, we explore a range of spherical self-consistent dynamical models with this light profile. We find that the corresponding deprojected volume density profile, phase space density and line-of-sight velocity distribution of these models are well fit by simple analytic approximations. We illustrate the application of these models by fitting a sample of about 25 galactic nuclei observed by Hubble Space Telescope. We give the derived volume density, phase space density, vel…
Observations of warm absorbers provided new ways to study the nuclear environments of AGNs. I discuss basic properties of warm absorbers and early developments with ROSAT, ASCA and HST observations. I briefly touch upon recent advances made with Chandra and FUSE.
The 2006 meeting in Xi'an on the Central Engine of Active Galactic Nuclei covered the enormous and continuously expanding area of AGN research, from theory to the most sophisticated observations and from gamma-ray energies to long radio wavelengths. This short summary gives some, but definitely not all, highlights and new results presented by the participants.
We present the general properties of the Active Galactic Nuclei (AGNs) and discuss the origin and structure of jets that are associated to a fraction of these objects. We then we address the problems of particle acceleration at highly relativistic energies and set limits on the luminosity of AGN jets for being origin of UHECRs.
A significant correlation between FeKalpha line energy and X-ray spectral slope has been discovered among radio-quiet active galactic nuclei. The ionization stage of the bulk of the FeKalpha emitting material is not the same in all active galactic nuclei and is related to the shape of the X-ray continua. Active galactic nuclei with a steep X-ray spectrum tend to have a fluorescence FeKalpha line from highly ionized material. In the narrow-line Seyfert 1 galaxies with steeper X-ray spectrum (Gamma_X > 2.1), the FeKalpha line originates from highly ionized material. In the Seyfert 1 galaxies and…
The Baldwin Effect, a negative correlation between emission-line equivalent width and luminosity in active galactic nuclei, is still of interest as a diagnostic of accretion physics nearly thirty years after its discovery. This review examines recent developments in the study of correlations between line and continuum emission in AGNs, as measured both in ensembles and in individual sources.
Recently Umemura, Fukue, & Mineshige (1997) proposed the radiative avalanche fueling to active galactic nuclei; gas accretion is driven by radiation drag exerted by stellar radiation from circumnuclear starburst regions. This mechanism is also interesting in terms of starburst-AGN connections. We therefore present observational tests for the radiative avalanche fueling. Our tests, however, show that gas accretion rates driven by the radiative avalanche are significantly lower than those expected from the standard accretion theory applied for typical active galactic nuclei with the circumnuclea…
Based on RXTE scans and observations with the SWIFT/XRT telescope and INTEGRAL observatory, we report the identification of four X-ray sources discovered during the RXTE Slew Survey of the |b|>10deg sky with nearby (z ~ 0.017-0.098) luminous (log L_2-10keV ~ 42.7-44 erg/s) active galactic nuclei. Two of the objects exhibit heavily intrinsically absorbed X-ray spectra (NHL~10^23 cm^-2).
This paper summarizes the observations which provide the best evidence for the presence of black holes in active galactic nuclei. This includes: X-ray variability; kinematical studies using optical emission lines as well as the distribution of megamaser spots; and the shape of the Fe K$α$ X-ray emission line. It also presents the current status of our understanding of jet-dominated active galaxies (blazars), and briefly reviews the currently popular AGN ``Unification Schemes.''
We derive the mass function of the relic black holes and compared with that of the Massive Dark Objects in galaxies. Under the assumption that accretion onto massive BH's powers the Active Galactic Nuclei, the mass function of the BH responsibile for the past activity of QSO/AGN is computed. Our results support the scenario in which the QSO phase has exclusively occurred in every proto-elliptical.
Cosmic ray (CR) acceleration at the shock created by the expanding cocoons around active galactic nuclei (AGNs) is studied. It is shown that above the energy $10^{18}$ eV the overall energy spectrum of CRs, produced during the AGN evolution and released in the intergalactic space, has the form $N\propto ε^{-γ}$, with $γ\approx 2.6$, which extends up to $ε_{max}\sim 10^{20}$ eV. It is concluded that cocoons shocks have to be considered as a main source of extragalactic CRs, which together with Galactic supernova remnants provide the observed CR spectrum.
High-resolution imaging and long-slit spectroscopy obtained with HST, combined with ground-based integral-field spectroscopy, provides the kinematics of stars and gas in nearby galactic nuclei with sufficient accuracy to derive the intrinsic dynamical structure, and to measure the mass of the central black hole. This has revealed that many nuclei contain decoupled kinematic components and asymmetric structures, and that nuclear and global properties of galaxies are correlated. Higher spatial resolution and significantly increased sensitivity are required to cover the full range of galaxy prope…
Extremely flat and inverted radio spectra as observed in galactic nuclei and BL Lac sources are still a challenge for fast particle acceleration models. Continuous acceleration by electric fields in reconnection regions can result in almost constant particle distributions and thereby in inverted synchrotron spectra independent from the details of the injected spectrum. These spectra are calculated from a spatially averaged diffusion transport equation in momentum space that includes systematic momentum gains and losses as well as finite particle escape lifetimes.
We consider the emission of high energy to very high energy $γ$-rays in radio-quiet active galactic nuclei (AGN) or the central regions of radio-loud AGN. We use our results to estimate the $γ$-ray flux from the central regions of nearby AGN, and then to calculate the contribution to the diffuse $γ$-ray flux from unresolved AGN.
Continuum and emission-line variability of active galactic nuclei provides a powerful probe of microarcsecond scale structures in the central regions of these sources. In this contribution, we review basic concepts and methodologies used in analyzing AGN variability. We develop from first principles the basics of reverberation mapping, and pay special attention to emission-line transfer functions. We discuss application of cross-correlation analysis to AGN light curves. Finally, we provide a short review of recent important results in the field.
The production and propagation of light antimatter nuclei has been calculated using inclusive antiproton production cross sections from a new data analysis, and coalescence models for the production of composite particles. Particles were propagated using recently proven phenomenological approaches. The atmospheric secondary flux is evaluated for the first time. The Galactic flux obtained are larger than those obtained previously in similar calculations. The non-annihilating scattering contributions of the propagated particles are introduced. The preliminary results are shown and discussed.
In this paper I summarize the science motivations, as well as a few mid-infrared spectroscopic methods used to identify the principal mechanisms of energy production in dust enshrouded galactic nuclei. The development of the various techniques is briefly discussed. Emphasis is given to the use of the data which are becoming available with the infrared spectrograph (IRS) on Spitzer, as well as the results which have been obtained by IRS over the past two years.
We show that the observational data of extragalactic radio sources tend to support the theoretical relationship between the jet precession period and the optical luminosity of the sources, as predicted by the model in which an accretion disk causes the central black hole to precess.
A magnetically-structured accretion disk corona, generated by buoyancy instability in the disk, can account for observations of flare--like events in Active Galactic Nuclei. We examine how Petschek magnetic reconnection, associated with MHD turbulence, can result in a violent release of energy and heat the magnetically closed regions of the corona up to canonical X-ray emitting temperatures. X-ray magnetic flares, the after effect of the energy released in slow shocks, can account for the bulk of the X-ray luminosity from Seyfert galaxies and consistently explain the observed short-timescale v…
There is growing evidence that relativistic jets in active galactic nuclei undergo extended (parsec-scale) acceleration. We argue that, contrary to some suggestions in the literature, this acceleration cannot be purely hydrodynamic. Using exact semianalytic solutions of the relativistic MHD equations, we demonstrate that the parsec-scale acceleration to relativistic speeds inferred in sources like the radio galaxy NGC 6251 and the quasar 3C 345 can be attributed to magnetic driving. Additional observational implications of this model will be explored in future papers in this series.
We have observed three Type 2 radio loud active galactic nuclei using the VLBA at five frequencies between 1.6 GHz and 15 GHz, in a search for the free-free absorption cutoff predicted by some models for optically-obscuring dusty molecular tori. We find no evidence for sharp cutoffs toward low frequencies in any of the sources. However, one source, OW637, has VLBI components which are steeply falling toward high frequencies (S_nu \propto nu^-2).
Massive black holes (MBHs) with a mass below ~ 1e7 Msun are likely to reside at the centre of dense stellar nuclei shaped by 2-body relaxation, close interactions with the MBH and direct collisions. In this contribution, we stress the role of mass segregation of stellar-mass black holes into the innermost tenths of a parsec and point to the importance of hydrodynamical collisions between stars. At the Galactic centre, collisions must affect giant stars and some of the S-stars.
We report the first results of a program to measure accurate stellar velocity dispersions in the bulges of the host galaxies of active galactic nuclei (AGNs) for which accurate black hole (BH) masses have been determined via reverberation mapping. We find good agreement between BH masses obtained from reverberation mapping, and from the M(BH) - sigma relation as defined by quiescent galaxies, indicating a common relationship between active and quiescent black holes and their large-scale environments.
We have used the STOKES radiative transfer code, to model polarization induced by dust scattering in the polar regions of Active Galactic Nuclei (AGN). We discuss the wavelength-dependence of the spectral intensity and polarization over the optical/UV range at different viewing angles for two different types of dust: a Galactic dust model, and a dust model inferred from extinction properties of AGN. The STOKES code and documentation are freely available at http://www.stokes-program.info/.
The Tartarus database contains products for 529 ASCA observations of active galactic nuclei. We have been updating Tartarus to include observing sequences conducted after 1999 January. The revised database will contain products for 375 objects, with a total of 614 observing sequences. We have begun a systematic timing analysis of the Tartarus light curves. We present here some preliminary results of an investigation into the relation between excess variance and black-hole mass. Having optimised our analysis to minimize the scatter in the variance measurements, we find that the narrow-line acti…
Recent results from a multiwavelength survey of spatially resolved outflows in nearby active galaxies are presented. Optical Fabry-Perot and long-slit spectroscopic data are combined with VLA and ROSAT images, when available, to probe the warm, relativistic and hot gas components involved in the outflow. The emphasis is put on objects which harbor wide-angle galactic-scale outflows but also show evidence at radio or optical wavelengths for collimated jet-like phenomena (e.g., Circinus, NGC 4388, and to a lesser extent NGC 2992). Our results are compared with the predictions from published jet-…
M87 is the first extragalactic source detected at the TeV that is not a blazar. To account for the recent observations of M87 made by the High Energy Stereoscopic System (H.E.S.S.) telescope array, we developed a new multi-blob synchrotron self-Compton model. In the framework of this model, we present here the predictions for the very high energy emission of three active galactic nuclei with extended optical or X-ray jets which could be misaligned blazars, namely Cen A, PKS 0521-36 and 3C 273.
We observed 37 AGN candidates and classified them on the basis of their spectroscopic properties; three are confirmed QSOs, one is a BL Lac object, nine are Seyfert 1 galaxies, four Seyfert 2s, while twenty are HII regions.
We discuss different aspects of nested bar dynamics and its effect on the gas flow and fueling of Active Galactic Nuclei. Specifically we focus on the dynamical decoupling between the primary and secondary bars and the gas flow across the bar-bar interface. We analyze the nuclear gaseous bar formation when gas gravity can be neglected or when it dominates. Finally, we discuss the possible effect of flat core, triaxial, dark halos on the formation of galactic bulges and supermassive black holes (SBHs) and argue in favor of SBH-bulge-halo correlation.
In our recent paper (Salucci et al. 1998) we have investigated the mass distribution function of massive dark objects in galaxies, exploiting the available optical and radio observations. Under the assumption that massive black holes power active galactic nuclei, we have compared the mass functions of massive dark objects and black holes responsible for the observed activity. We have found that a scenario with a single short burst per active galactic nucleus is in a good agreement with the available data. Here we summarize and discuss the main points of our study.
Objective-prism surveys for UV-excess and emission-line objects, especially the First and Second Byurakan Surveys, have been central to the study of active galactic nuclei (AGNs). We review previous line-selected surveys for AGNs and discuss their contribution to our understanding of the AGN phenomena. In addition, we present results from the KPNO International Spectroscopic Survey, a modern digital objective-prism survey for emission line objects. This survey is discovering substantial numbers of new AGNs, in particular low-luminosity AGNs and LINERs.
We consider the role of centrifugal force on the energetics of electrons moving along the magnetic field lines of spinning active galactic nuclei. We find the energy gained by charged particles against inverse Compton scattering and/ synchrotron radiation losses, become quite significant in the region close to the light cylinder. The particles accelerated by the centrifugal force become a part of the jet material. The scattering of UV-photons against the energetic electrons lead to the generation of X-ray and gamma-ray photons within the light cylinder, and outside, they radiate the non-therma…
I propose a new mechanism for explanation of nonthernal X-ray emission of large-scale AGN jets. Namely, I assume that this radiation has synchrotron origin emitted by extremely high energy protons, and discuss implications of this model for several prominent hot spots and knots resolved by Chandra in Pictor A, 3C 120, PKS 0637-752, 3C 273.
Relativistic outflows are a common phenomenon in accreting black holes. Despite the enormous differences in scale, stellar-mass black holes in binaries and supermassive black holes in Galactic Nuclei produce jets with analogous properties. In both are observed two types of relativistic outflows: 1) steady compact jets with flat-spectrum, and 2) sporadic extended jets with steep-spectrum and apparent superluminal motions. Besides, the most common class of gamma-ray bursts are afterglows from ultra-relativistic jets associated to the formation of black holes at cosmological distances.
We examine all possible stationary, optically thick, geometrically thin accretion disc models relevant for active galactic nuclei (AGN) and identify the physical regimes in which they are stable against the thermal-viscous hydrogen ionization instability. Self-gravity and irradiation effects are included. We find that most if not all AGN discs are unstable. Observed AGN therefore represent the outburst state, although some or all quasars could constitute a steady population having markedly higher fuelling rates than other AGN. It has important implications for the AGN mass supply and for the p…
The decay of globular clusters to the center of their mother galaxy corresponds to carrying a quantity of mass sufficient to sustain the gravitational activity of a small pre-existing nucleus and to accrete it in a significant way. This is due to both dynamical friction of field stars and tidal disruption by the compact nucleus. The results of the simplified model presented here show that the active galactic nuclei luminosity and lifetime depend on the characteristics of the globular cluster system and are quite insensitive to the nucleus' initial mass.
We investigated the correlation between nuclear/circumnuclear starbursts around the active galactic nuclei (AGNs) and the AGN activities for 43 Seyfert galaxies in the CfA and 12 micron samples. We found that circumnuclear starburst luminosity as well as nuclear starburst luminosity are positively correlated with AGN luminosity. Moreover, nuclear starburst luminosity is more strongly correlated with the AGN luminosity normalized with AGN Eddington luminosity than is circumnuclear starburst luminosity. This implies that starbursts nearer the AGN could have a greater effect on AGN mass accretion…
Parsec scale jet properties are shortly presented and discussed. Observational data are used to derive constraints on the jet velocity and orientation, the presence of velocity structures, and the connection between the pc and kpc scale. Two peculiar sources with limb-brightened jets: 1144+35 and Mkn 501 are discussed in detail.
We present a progress report on a project whose goal is to improve both the precision and accuracy of everberation-based black-hole masses. Reverberation masses appear to be accurate to a factor of about three, and the black-hole mass/bulge velocity dispersion relationship appears to be the same in active and quiescent galaxies.
These lectures take a look at how observations with adaptive optics (AO) are beginning to influence our understanding of active galactic nuclei (AGN). By focussing on a few specific topics, the aim is to highlight the different ways in which enhanced spatial resolution from AO can aid the scientific analysis of AGN data. After presenting some background about how AO works, I will describe a few recent observations made with AO of QSO host galaxies, the Galactic Center, and nearby AGN, and show how they have contributed to our knowledge of these enigmatic objects.
A gap in phase space is opened up by a binary supermassive black hole as it ejects stars in a galactic nucleus. This gap must be refilled before the single black hole that subsequently forms can disrupt or accrete stars. We compute loss cone refilling rates for a sample of elliptical galaxies as a function of the mass ratio of the binary. Refilling times are of order 10 Gyr or longer in bright elliptical galaxies. Tidal flaring rates in these galaxies might be much lower than predicted using time-independent models.
