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    Laura FerrareseDepartment of Physics and Astronomy, Rutgers UniversityPiscataway, NJ 08854 USA

    Holland FordDepartment of Physics and Astronomy, Johns Hopkins UniversityBaltimore, Maryland 21218 USA

    May 11, 2004


    This review discusses the current status of supermassive black hole research, asseen from a purely observational standpoint. Since the early 90s, rapid technologicaladvances, most notably the launch of the Hubble Space Telescope, the commission-ing of the VLBA and improvements in near-infrared speckle imaging techniques,have not only given us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of the centralsingularity and the global properties of the host galaxy. It is thanks to these obser-vations that we are now, for the first time, in a position to understand the origin,evolution and cosmic relevance of these fascinating objects.


    The menagerie of Active Galactic Nuclei (AGNs) is as eclectic as couldbe imagined. Quasars, radio galaxies, Seyfert nuclei, Blazars, LINERS,BL-Lac objects, to name a few, are set apart from each other both bythe detailed character of the activity which takes place in the nuclei, andby the traits of the galaxies which host them. Underneath this appar-ent diversity, however, lie three revealing common properties. First,AGNs are extremely compact. Flux variability a staple of all AGNs confines AGNs to within the distance light can travel in a typicalvariability timescale. In many cases, X-ray variability is observed ontime scales of less than a day, and flares on time scales of minutes(e.g. NGC 6814, Tennant & Mushotzky 1983; MCG 6-30-15, McHardy1988). Second, the spectral energy distribution is decisively non-stellar:roughly speaking, AGNs power per unit logarithmic frequency intervalis constant over seven decades in frequency, while stars emit nearly allof their power in a frequency range a mere factor three wide. Third,AGNs must be very massive, a conclusion supported by two indepen-dent arguments. AGNs bolometric luminosities are astoundingly large:at least comparable, and often several orders of magnitude larger than

  • 2 Laura Ferrarese and Holland Ford

    the luminosity of the entire surrounding galaxy. Masses in excess of 106M are needed for an AGN not to become unbound by its ownoutpouring of energy. Furthermore, according to our best estimates,AGNs remain active for upward of 107 years: during this period, anenormous amount of material, well over a million solar masses, mustbe consumed to sustain their luminosity, even assuming a very highefficiency of energy production.

    Taken together, these considerations lead to the inescapable conclu-sion that the source of the nuclear activity is accretion onto a central,supermassive black hole (SBH; Rees 1984). Indeed, evidence of a rel-ativistic regime is betrayed, at least in some AGNs, by superluminalmotions of the radio jets, and by the broadening of low excitation X-ray emission lines (see 8.2). In our standard picture, the accretedmatter is thought to be confined in an accretion disk, or more gener-ally optically thick plasma, glowing brightly at ultraviolet (UV) andperhaps soft X-ray wavelengths. Medium and hard X-ray emission isproduced by inverse Compton scattering in a corona of optically thinplasma which might surround or sandwich the disk. Clouds of line-emitting gas move at high velocity around this complex core and are inturn surrounded by an obscuring torus or warped disk of gas and dust,with a sea of electrons permeating the volume within and above thetorus. What is commonly referred to as the AGN paradigm states thatthe detailed character of the nuclear activity can always be reproducedby finely tweaking, rather than completely revising, this basic picture.Changes in the angle at which the AGN is observed, in the spin and/ormass of the black hole, in the accretion rate, and in the modalities withwhich the surrounding interstellar medium interacts with the emergingAGN flux, account for the varied types found in the AGN zoo.

    Although the black hole paradigm originated and evolved exclu-sively within the AGN context, modern SBH searches have targetedalmost exclusively quiescent or weakly active nearby galaxies - and itson these galaxies that this review will mainly focus. There are two goodreasons for this. First, dormant SBHs are expected to be found in thenuclei of quiescent galaxies. The cumulative SBH mass density neededto explain the energetics of high redshift powerful quasars falls short,by at least two orders of magnitudes, to the one required to powerlocal AGNs (Padovani, Burg & Edelson 1990; Ferrarese 2002a). Theunaccounted SBHs must therefore reside in local, quiescent galaxies.Second, the telltale Keplerian dynamical signature imprinted by a cen-tral compact object on the motion of the surrounding gas and stars canonly be resolved in the most nearby galactic centers and, unfortunately,most nearby galaxies are not powerful AGNs. In passing, it must bementioned that although it is now accepted that SBH are present in

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    the nuclei of quiescent galaxies, we still do not completely understandhow the two coexist; in view of the abundant supply of gas and dustin galactic centers, preventing a SBH from accreting and immediatelyproducing an AGN is not a simple task (Fabian & Canizares 1988).Indeed, a definitive answer to this dilemma has yet to be found (Reeset al. 1982; Narayan & Yi 1994; Blandford & Begelman 1999; di Matteoet al. 1999).

    The most recent review on SBHs was published in 1995 (Kormendy& Richstone 1995). Since then, progress in this field has been so rapidthat any attempt to summarize it was destined to be outdated by pub-lication time. The number of local SBH detections has gone from a fewin 1995 to almost three dozens in 2004. Strong connections betweenSBHs and their host galaxies have emerged. Formation and evolution-ary scenarios have become more tightly constrained. After such feverishactivity, we are now at a turning point, when progress is once againslowing down as observational facilities are been exploited to their limit.

    This review will be concerned exclusively with supermassive blackholes. There is controversial evidence that intermediate mass blackholes (IBHs), bridging the gap between the stellar mass (a few to a fewtens of solar masses) and supermassive (over a million solar masses)varieties, might exist in the off-nuclear regions of some star-forminggalaxies and perhaps at the centers of globular clusters. An excellentreview of intermediate mass black holes is given by Miller & Colbert(2004), and we will not discuss the issue any further. Some useful for-malism will be introduced in 2. We will then present a brief historicaloverview of the subject ( 3). Although most reviews dispense withit, the history of SBHs is a fascinating example of the long trail oftentative steps, missed clues, and heterogeneous research areas whichultimately need to congeal for seemingly unforeseen and revolutionaryideas to emerge. Readers who are familiar with this history are invitedto skip to 4; readers desiring a comprehensive history of the theoreti-cal developments should refer to Thornes book Black Holes and TimeWarps, Einsteins Outrageous Legacy (1994). We will then move on todiscuss the several methods which can be used to measure SBH masses( 4- 8), with particular emphasis on resolved stellar and gas dynamicalstudies carried out with the Hubble Space Telescope. Scaling relation,linking SBH masses to the overall properties of the host galaxies, arediscussed in 10. SBH demographics, from high redshift quasars tolocal galaxies, is discussed in 10. Finally, in 11 we will discuss themost pressing open questions and the issues on which future progressis most likely to be made.

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    For convenience, we present in this section some terminology and equa-tions which will recurr in the remainder of this review.

    An important measure of the accretion rate onto a BH of massM is provided by the Eddington luminosity, i.e. the luminosity atwhich radiation pressure on free electrons balances the force of gravity.Because the force due to radiation ,ure has exactly the same inversesquare dependence on distance as gravity, but does not depend on mass,LE is independent of distance but depends on M:

    LE =4GMmpc

    T 1.3 1046




    erg s1 (1)

    where mp is the proton rest mass and T is the Thomson cross sec-tion. Above the Eddington luminosity, the source is unable to maintainsteady spherical accretion (although the presence of magnetic fields canconsiderably complicate the picture, Begelman 2001).

    Related to the Eddington luminosity is the Salpeter time

    tS =T c

    4Gmp 4 108 yr (2)

    defined as the time it would take a black hole radiating at the Edding-ton luminosity to dissipate its entire rest mass. is the efficiency ofconversion of mass into energy, and depends on the spin of the blackhole, varying between 6% if the black hole is not spinning, and 42% ifthe black hole is maximally spinning.

    The boundary of a (non-rotating) black hole of mass M is aspherical surface called the event horizon, the radius of which is givenby the Schwarzschild (or gravitational) radius:

    rSch =2GM

    c2 3 1013




    cm 2




    A.U. (3)

    At the Schwarzschild radius the gravitational time dilation goes toinfinity and lengths are contracted t

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