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eaa.iop.org DOI: 10.1888/0333750888/2365 Supermassive Black Holes in Active Galactic Nuclei Luis C Ho, John Kormendy From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Downloaded on Thu Mar 02 23:39:17 GMT 2006 [131.215.103.76] Institute of Physics Publishing Bristol and Philadelphia Terms and Conditions

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Page 1: Supermassive Black Holes in Active Galactic Nucleigeorge/ay21/eaa/eaa-agn-smbh.pdf · Supermassive Black Holes in Active Galactic Nuclei ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS

eaa.iop.orgDOI: 10.1888/0333750888/2365

Supermassive Black Holes in Active Galactic NucleiLuis C Ho, John Kormendy

From

Encyclopedia of Astronomy & AstrophysicsP. Murdin

© IOP Publishing Ltd 2006

ISBN: 0333750888

Downloaded on Thu Mar 02 23:39:17 GMT 2006 [131.215.103.76]

Institute of Physics PublishingBristol and Philadelphia

Terms and Conditions

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Supermassive Black Holes in Active Galactic Nuclei ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS

QUASARS are among the most energetic objects in the uni-verse. We now know that they live at the centers of galax-ies and that they are the most dramatic manifestation ofthe more general phenomenon of ACTIVE GALACTIC NUCLEI

(AGNs). These include a wide variety of exotica such asSYEFERT GALAXIES, RADIO GALAXIES and BL LACERTAE OBJECTS.Since the discovery of quasars in 1963, much effort hasgone into understanding their energy source. The suite ofproposed ideas has ranged from the relatively prosaic,such as bursts of star formation that make multiplesupernova explosions, to the decidedly more colorful,such as supermassive stars, giant pulsars or ‘spinars’,and supermassive black holes (SMBHs). Over time,SMBHs have gained the widest acceptance. The keyobservations that led to this consensus are as follows.

Quasars have prodigious luminosities. Not uncom-monly, L~1046 erg s–1; this is 10 times the luminosity ofthe brightest galaxies. Yet they are tiny, because they varyon timescales of hours. From the beginning, the need foran extremely compact and efficient engine could hardlyhave been more apparent. Gravity was implicated,because collapse to a BLACK HOLE is the most efficientenergy source known. The most cogent argument is dueto Donald Lynden-Bell (1969). He showed that anyattempt to power quasars by nuclear reactions alone isimplausible. First, a lower limit to the total energy outputof a quasar is the energy, ~1061 erg, that is stored in itsradio-emitting plasma halo. This energy weighs 1040 g or107Mo

. . However, nuclear reactions produce energy withan efficiency of only ε=0.7%. Then the waste mass leftbehind in powering quasars would be at least M•~–109Mo

. .Lynden-Bell argued further that quasar engines are2R~~1015 cm in diameter because large variations inquasar luminosities are observed on timescales as shortas 10 h. However, the gravitational potential energy of109Mo

. compressed into a volume as small as 10 light-hours is ~GM•2</R>~1062 erg. As Lynden-Bell noted,‘Evidently although our aim was to produce a modelbased on nuclear fuel, we have ended up with a modelwhich has produced more than enough energy by gravi-tational contraction. The nuclear fuel has ended as anirrelevance’. We now know that the total energy outputis larger than the energy that is stored in a quasar’s radiosource; this strengthens the argument. Meanwhile, acaveat has appeared: the objects that vary most rapidlyare now thought to contain relativistic jets that arebeamed at us. This boosts the power of a possibly smallpart of the quasar engine and weakens the argument thatthe object cannot vary on timescales less than the lighttravel time across it. However, this phenomenon wouldnot occur at all if relativistic motions were not involved,

so SMBH-like potential wells are still implicated. Theseconsiderations suggest that quasar power derives fromgravity.

The presence of deep gravitational potentials haslong been inferred from the large velocity widths of theemission lines seen in optical and ultraviolet spectra ofAGNs. These are typically 2000–10000 km s–1. If the largeDoppler shifts arise from gravitationally bound gas, thenthe binding objects are both massive and compact. Theobstacle to secure interpretation has always been therealization that gas is easy to push around: explosionsand ejection of gas are common astrophysical phen-omena. The observation that unambiguously points torelativistically deep gravitational potential wells is thedetection of radio jets with plasma knots that are seen tomove faster than the speed of light, c. Apparent expan-sion rates of (1–10)c are easily achieved if the true expan-sion rate approaches c and the jet is pointed almost at us.

The final pillar on which the SMBH paradigm isbased is the observation that many AGN jets are well collimated and straight. Evidently AGN engines canremember ejection directions with precision for up to 107 yr. The natural explanation is a single rotating bodythat acts as a stable gyroscope. Alternative AGN enginesthat are made of many bodies—such as stars and super-novae—do not easily make straight jets.

A variety of other evidence also is consistent withthe SMBH picture, but the above arguments were theones that persuaded a majority of the astronomical com-munity to take the extreme step of adopting SMBHs asthe probable engine for AGN activity. In the meantime,SMBH alternatives such as single supermassive stars andspinars were shown to be dynamically unstable andhence short lived. Even if such objects can form, they arebelieved to collapse to SMBHs.

The above picture became the paradigm long beforethere was direct evidence for SMBHs. Dynamical evi-dence is the subject of this article and the following one(SUPERMASSIVE BLACK HOLES IN ACTIVE GALAXIES).Meanwhile, there are new kinds of observations thatpoint directly to SMBH engines. In particular, recentobservations by the Advanced Satellite for Cosmologyand Astrophysics (ASCA) have provided strong evidencefor relativistic motions in AGNs. The x-ray spectra ofmany Seyfert galaxy nuclei contain iron Kα emissionlines (rest energies of 6.4–6.9 keV; see figure 1). Theselines show enormous Doppler broadening—in somecases approaching 100 000 km s–1 or 0.3c—as well asasymmetric line profiles that are consistent with rela-tivistic boosting and dimming in the approaching andreceding parts, respectively, of SMBH accretion disks assmall as a few Schwarzschild radii.

The foregoing discussion applies to the most pow-erful members of the AGN family, namely quasars andhigh-luminosity Seyfert and radio galaxies. It is less

Supermassive Black Holes in ActiveGalactic Nuclei

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compelling for the more abundant low-luminosityobjects, where energy requirements are less demandingand where long jets or superluminal motions are seenless frequently or less clearly. Therefore a small butvocal competing school of thought continues to arguethat stellar processes alone, particularly those that occurduring bursts of STAR FORMATION, can reproduce manyAGN characteristics. Nonetheless, dynamical evidencesuggests that SMBHs do lurk in some mildly activenuclei, and, as discussed in the next article, even in themajority of inactive galaxies.

Measuring AGN masses: direct methodsVery general arguments suggest that quasar engineshave masses M•~106–109Mo

. . Gravitational collapse isbelieved to liberate energy with an efficiency of ε~–0.1;Lynden-Bell’s arguments then imply that typical rem-nant masses are M•~108Mo

. . Better estimates can bederived by asking what we need in order to powerquasar luminosities, which range from 1044 to 1047 erg s–1

or 1011Lo. to 1014Lo

. . For ε=0.1, the engine must consume0.02–20Mo

. yr–1. How much waste mass accumulatesdepends on how long quasars live. This is poorly known.If they live long enough to make radio jets that are colli-mated over several Mpc, and if their lifetimes are conser-vatively estimated as the light travel time along the jets, then quasars last >~107 yr and reach massesM•>~105–108Mo

. . However, the most rigorous lower limiton M• follows from the condition that the outward forceof radiation pressure on accreting matter not overwhelmthe inward gravitational attraction of the engine, a con-dition which, admittedly, strictly holds only if the accret-ing material and the radiation have spherical symmetry.This so-called Eddington limit requires that L<– LE~(4πιGcmp/σT)M•=1.3×1038 (M•/Mo

. ) erg s–1, or

Copyright © Nature Publishing Group 2002Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2002Dirac House, Temple Back, Bristol, BS21 6BE, UK 2

Figure 1. A composite x-ray spectrum of Seyfert nuclei takenwith ASCA showing the relativistically broadened Fe Kα line.The solid curve is a fit to the line profile using two Gaussians, anarrow component centered at 6.4 keV and a much broader, red-shifted component. (Figure adapted from Nandra K et al 1997Astrophys. J. 477 602.)

Figure 2. (a) HST image of the ionized gas disk near the center of the giant elliptical galaxy M87. The data were takenwith the Second Wide Field/Planetary Camera through a filter that isolates the optical emission lines Hα and [N II] λλ6548, 6583. The left inset is an expanded view of the gas disk; for an adopted distance of 16.8 Mpc, the region shown is5´´×5´´ or 410 pc×410 pc. The disk has a major axis diameter of ~150 pc, and it is oriented perpendicular to the opticaljet. (Image courtesy of NASA/Space Telescope Science Institute, based on data originally published by Ford H C et al1994 Astrophys. J. 435 L27.) (b) Optical emission-line rotation curve for the nuclear disk in M87. The data were takenwith the Faint Object Camera on HST. The curves in the upper panel correspond to two different Keplerian thin-diskmodels, and the bottom panel shows the residuals for the best-fitting model. (Figure adapted from Macchetto F et al1997 Astrophys. J. 489 579.)

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equivalently that M•>– 8×105(L/1044 erg s–1)Mo.. Here G is

the gravitational constant, mp is the mass of the protonand σT is the Thompson cross section for electron scat-tering. We conclude that we are looking for SMBHs withmasses M•~106–109Mo

. . Finding them has become one ofthe ‘Holy Grails’ of astronomy because of the importanceof confirming or disproving the AGN paradigm.

AGNs provide the impetus to look for SMBHs, butACTIVE GALAXIES are the most challenging huntingground. Stellar dynamical searches first found centraldark objects in inactive galaxies (see SUPERMASSIVE BLACK

HOLES IN INACTIVE GALAXIES), but they cannot be appliedin very active galaxies, because the nonthermal nucleusoutshines the star light. We can estimate masses usingthe kinematics of gas, but only if it is unperturbed bynongravitational forces. Fortunately, this complicationcan be ruled out a posteriori if we observe that the gas isin Keplerian rotation around the center, i.e. if its rotationvelocity as a function of radius is V(r):r–1/2. We can alsostack the cards in our favor by targeting galaxies that areonly weakly active and that appear to show gas disks inimages taken through narrow bandpasses centered onprominent emission lines.

Kinematics of optical emission linesHigh-resolution optical images taken with ground-basedtelescopes and especially with the Hubble SpaceTelescope (HST) show that many giant ELLIPTICAL GALAX-IES contain nuclear disks of dust and ionized gas. Themost famous case is M87 (figure 2(a)). The disk measures~150 pc across, and its rotation axis is closely alignedwith the optical and radio jet. This is in accord with theSMBH accretion picture. The disk is in Keplerian rotationfigure 2(b)) around an object of mass M•~–3×109Mo

. .Furthermore, this object is dark: the measured mass-to-light ratio exceeds 100 in solar units, and this is muchlarger than that of any known population of stars.Moreover, the dark mass must be very compact: thevelocity field limits its radial extent to be less than 5 pc.Therefore its density exceeds 107Mo

. pc–3. Another illus-tration of this technique is given in figure 3. M84, also adenizen of the Virgo cluster of galaxies, is a twin of M87in size, and it, too, harbors an inclined nuclear gas disk(diameter ~80 pc), whose rotation about the centerbetrays an invisible mass of M•~–2×109Mo

. . Other casesare reported (NGC 4261, NGC 6251, NGC 7052), andsearches for more are in progress.

Kinematics of radio masersA related approach exploits the few cases where 22 GHzmicrowave maser emission from water molecules hasbeen found in edge-on nuclear disks of gas. Particularlystrong ‘megamasers’ allow radio astronomers to useinterferometry to map the velocity field with exquisiteangular resolution. In the most dramatic application ofthis method, the Very Long Baseline Array was used toachieve resolution 0´´.0006—100 times better than thatdelivered by HST—in observations of the Seyfert galaxyNGC 4258. This is only 6 Mpc away, so the linear resolu-tion was a remarkable 0.017 pc. The masers trace out aslightly warped annulus with an inner radius of 0.13 pc,an outer radius of 0.26 pc and a thickness of <0.003 pc(figure 4, left). The masers with nearly zero velocity withrespect to the galaxy are on the near side of the diskalong the line of sight to the center, while the featureswith high negative (approaching) and positive (receding)velocities come from the disk on either side of the center.High velocities imply that 3.6×107Mo

. of binding matterresides interior to r=0.13 pc. What is most compellingabout NGC 4258 is the observation that the rotationcurve is so precisely Keplerian (figure 4, right). From thisresult, one can show that the radius of the mass distribu-tion must be r<~0.012 pc. If the central mass were not anSMBH, its density would be extraordinarily high,ρ;>5×1012Mo

. pc–3. This is comparable with the density ofthe dark mass at the center of our Galaxy (see SUPERMAS-SIVE BLACK HOLES IN ACTIVE GALAXIES). Under theseextreme conditions, one can show that a cluster of stellar

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Figure 3. (Left) HST image of the central region of thegiant elliptical galaxy M84; the box measures 22´´×19´´ or1.8 kpc×1.6 kpc for an adopted distance of 16.8 Mpc. Thedata were taken with the Second Wide Field/PlanetaryCamera through a filter that isolates the optical emissionlines Hα and [N II] λλ 6548, 6583. The slit of the SpaceTelescope Imaging Spectrograph was placed along themajor axis of the nuclear gas disk (blue rectangle).(Right) Resulting spectrum of the central 3´´ (240 pc). Theabscissa is velocity and the ordinate is distance along themajor axis. The spectrum shows the characteristic kine-matic signature of a rotating disk. The velocity scale iscoded such that blue and red correspond to blueshiftsand redshifts, respectively; the total velocity range is1445 km s–1. (Image courtesy of NASA/Space TelescopeScience Institute, based on data originally published byBower G A et al 1997 Astrophys. J. 483 L33 and Bower GA et al 1998 Astrophys. J. 492 L111.)

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remnants (white dwarf stars, neutron stars and stellar-size black holes) or substellar objects (planets and browndwarfs) is short lived. Astrophysically, these are the mostplausible alternatives to an SMBH. Therefore the dynam-ical case for an SMBH is stronger in NGC 4258 and in ourGalaxy than in any other object.

Measuring AGN masses: indirect methodsDirect dynamical measurements are impractical for moreluminous and more distant AGNs. The tremendous glarefrom the nucleus outshines the circumnuclear emissionfrom stars, and the violent conditions near the center arelikely to subject the gas to nongravitational forces.Indirect methods of estimating central masses have been

devised to provide a reality check for these more difficultobjects.

Fitting the spectra of accretion disksAs material falls toward a black hole, it is believed to set-tle into an ACCRETION DISK in which angular momentumis dissipated by viscosity. From the virial theorem, half ofthe gravitational potential energy U is radiated.Therefore the luminosity is

At sufficiently high accretion rates .

M•, the gas isoptically thick, and the disk radiates as a thermal black-body:

Copyright © Nature Publishing Group 2002Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998

Figure 4. (Left) Spatial distribution of the water masers in NGC 4258, with redshifted velocities to the left and blueshift-ed velocities to the right. The maser spots are distributed in a thin, warped annulus that is only 4° from edge on. Foran adopted distance of 6.4 Mpc, 1 mas=0.031 pc. The top panel shows an expanded view of the emission near the sys-temic velocity of the galaxy. (Right) Light-of-sight velocity as a function of distance along the major axis of the annu-lus. The high-velocity features are accurately fitted by a Keplerian model, overplotted as a continuous line. The emis-sion near the systemic velocity, magnified in the inset, lies at nearly constant radius in the front part of the disk alongthe line of sight to the center. The linear velocity gradient results from the change in projection of the rotation velocity.(Figure adapted from Miyoshi M et al 1995 Nature 373 127.)

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Here 2πιr2 is the surface area of the disk and σ is theStefan–Boltzmann constant. The effective temperature ofthe disk as a function of radius r is therefore

Parameterizing the above result in terms of theEddington accretion rate,

.ME~LE/εc2=2.2(ε/0.1)–1(M•/

108Mo.) Mo

. yr–1, and the Schwarzschild radius,RS~2GM•/c2=2.95×1013(M•/108Mo

. ) cm, gives

In other words, the peak of the blackbody spectrumoccurs at a frequency of νmax=2.8kT/h~–4×1016 Hz, wherek is Boltzmann’s constant and h is Planck’s constant. Thispeak is near 100 å or 0.1 keV. In fact, the spectra of manyAGNs show a broad emission excess at extreme ultravi-olet or soft x-ray wavelengths. This ‘big blue bump’ hasoften been identified with the thermal emission from theaccretion disk. A fit to the luminosity and the central fre-quency of the big blue bump gives M• and

.M• but not

each separately. Corrections for disk inclination and rela-tivistic effects further complicate the analysis. Thismethod is therefore model dependent and provides onlyapproximate masses. Typical values for quasars areM•~–(108–109.5)Mo

. and .

M•~–(0.1–1) .

ME. Seyfert nucleiappear to have lower masses, M•~–(107.5–108.5)Mo

. , andlower accretion rates,

.M•~–(0.01–0.5)

.ME.

Virial masses from optical variabilitySurrounding the center at a distance of 0.01–1 pc from theblack hole lies the ‘broad-line region’ (BLR). This is acompact, dense and highly turbulent swarm of gasclouds or filaments. The clouds are illuminated by theAGN’s photoionizing continuum radiation andreprocess it into emission lines that are broadened tovelocities of several thousand km s–1 by the strong grav-itational field of the black hole. Then

where η~–1–3 depends on the kinematic model adopted,v is the velocity dispersion of the gas as reflected in thewidths of the emission lines and rBLR is the radius of theBLR. The latter can be estimated by ‘reverberation map-ping’, as follows. The photoionizing continuum of anAGN typically varies on timescales of days to months. Inresponse, the emission lines vary also, but with a timedelay that corresponds to the light travel time betweenthe continuum source and the line-emitting gas. By mon-itoring the variations in the continuum and the emission

lines in an individual object, reverberation mapping pro-vides information on the size of the BLR. These studiesalso support the assumption that the line widths comepredominantly from bound orbital motions. Applyingequation (5) suggests that Seyfert nuclei are powered byblack holes with masses M•~(107–108)Mo

. , while quasarengines are more massive, with M•~(108–109)Mo

. . Sincequasars also live in more massive host galaxies, this sup-ports the emerging correlation (see the following article)between SMBH mass and the mass of the elliptical-galaxy-like part of the host galaxy.

X-ray variabilityAGNs vary most conspicuously in hard x-rays (2–10keV). One might hope to use the variability timescale toconstrain the size of the x-ray-emitting region and henceto estimate the central mass. However, no simple patternof variability emerges, and defining a meaningfultimescale is ambiguous. One approach uses the ‘fastestdoubling time’, ∆t, to establish a maximum source sizeR~–c ∆t. High-energy photons presumably come from thehot, inner regions of the accretion disk or in an overlyinghot corona. For example, if R~–5RS, as deduced in somemodels, we obtain an upper limit to the mass,M•<~(c3/10G) ∆t~104 ∆t Mo

. (∆t in s). Masses estimated inthis way are generally consistent with those obtainedfrom other virial arguments, but they are considerablyless robust because of uncertainties in associating thevariability timescale with a source size. For example, thex-ray intensity variations could originate from localized‘hotspots’ in the accretion flow.

X-ray reverberation mapping may in the future be amore powerful tool. The iron Kα line is widely believedto be produced by reprocessing of the hard x-ray contin-uum by the accretion disk. The strikingly large widthand skewness of the line profiles (figure 1), now routine-ly detected with ASCA, reflect the plasma bulk motionwithin 10–100 gravitational radii of the center. The tem-poral response of the line strength and line profiledepends on a number of factors that, in principle, can bemodeled theoretically; these include the geometry of thex-ray source, the structure of the disk, and the assumed(Schwarzschild or Kerr) metric of the black hole. Time-resolved X-ray spectroscopy should become feasiblewith the X-ray Multi-Mirror Mission (XMM) in the nearfuture. We can then look forward to constraints on boththe masses and the spins of SMBHs.

Summary and prospectshe black hole model for AGN activity has been success-ful and popular for over three decades. It has withstoodthe test of time not—at least until recently—because theempirical evidence for SMBHs has been overwhelmingbut because the alternatives are so implausible. Nowprogress has advanced on several fronts. The refurbished

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HST has greatly strengthened the evidence, alreadygrowing from ground-based observations, that super-massive dark objects live at the centers of most galaxies.The pace of discoveries is accelerating. The dark objectshave exactly the range of masses that we need to explainAGN engines, but we have had no proof that they mustbe black holes. Then radio interferometry revealed thespectacular maser disk in NGC 4258. For its rotationcurve to be as accurately Keplerian as we observe, thecentral mass must be confined to an astonishingly tinyvolume. The inferred density of the central object is sohigh that astrophysically plausible alternatives can beexcluded; an SMBH is the best explanation. The sameconclusion has been reached for the SMBH candidate atthe center of our Galaxy. This is a major conceptualbreakthrough.

In addition, ASCA has demonstrated that manyAGNs show iron emission lines with relativisticallybroadened profiles. This is arguably the best evidence forthe strong gravitational field of a black hole. One of themost interesting prospects for the future is time-resolvedx-ray spectroscopy, because hot gas probes closest to anaccreting black hole.

Finally, the AGN paradigm can be turned inside outto give what may prove to be the most direct argumentfor black holes. SMBHs were ‘invented’ to explainnuclear activity in galaxies. In recent years, an ironic sit-uation has developed: some SMBH candidates are tooinactive for the amount of matter that we believe they areaccreting. The same is true of some stellar-mass blackhole candidates that accrete gas from evolving compan-ion stars. A number of researchers recently have devel-oped a theory of ‘advection-dominated accretion’ inwhich the accretion disk cannot radiate most of its ener-gy before it reaches RS either because it is optically thickor because it is too thin to cool. Unless most of the inflow-ing material ultimately escapes through an outflow, apossibility being explored, the only way to make theaccretion energy disappear is to ensure that the accretingbody does not have a hard surface. That is, the inactivityof well-fed nuclear engines may be evidence that theyhave event horizons. Finding event horizons would bedefinitive proof that AGN engines are black holes.

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Copyright © Nature Publishing Group 2002Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2002Dirac House, Temple Back, Bristol, BS21 6BE, UK 6

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