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CALTECH AY 21 - Supermassive Black Holes in Active Galactic Nuclei

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eaa.iop.orgDOI: 10.1888/0333750888/2365 Supermassive Black Holes in Active Galactic NucleiLuis C Ho, John Kormendy FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:39:17 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsSupermassive Black Holes in Active Galactic NucleiENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSQUASARS 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~1046erg 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, ~1061erg, that is stored in itsradio-emitting plasma halo. This energy weighs 1040g 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~~1015cm 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>~1062erg. 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 107yr. 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–1or 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 lessSupermassive Black Holes in ActiveGalactic NucleiCopyright © 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, UK1Supermassive Black Holes in Active Galactic NucleiENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICScompelling for the more abundant low-luminosityobjects, where energy requirements are less


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