Stellar-Mass Black Holes and Ultraluminous X-ray Sources Rob Fender and Tomaso Belloni Science 337, 540 (2012); DOI: 10.1126/science.1221790
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Einstein equations to describe the black hole, one dissipative behavior in model systems with a de- transition amplitude
REVIEW both were moderately certain it was a black hole, Hawking wanted an insurance policy). In 1990, Hawking conceded the bet, accepting that the on March 28, 2013 Stellar-Mass Black Holes source contained a black hole. Since then, astron- omers have discovered many hundreds of x-ray and Ultraluminous X-ray Sources binaries within the Milky Way and beyond, several tens of which are good candidate BHXRBs (4). Rob Fender1* and Tomaso Belloni2 Over the past decade, repeating empirical pat- terns connecting the x-ray, radio, and infrared emis- We review the likely population, observational properties, and broad implications of stellar-mass sion from these objects have been found and used black holes and ultraluminous x-ray sources. We focus on the clear empirical rules connecting to connect these observations to physical compo-
accretion and outflow that have been established for stellar-mass black holes in binary systems in nents of the accretion flow (Fig. 1). It is likely that www.sciencemag.org the past decade and a half. These patterns of behavior are probably the keys that will allow us some of these empirical patterns of behavior also 5 9 to understand black hole feedback on the largest scales over cosmological time scales. apply to accreting supermassive (10 to 10 M⊙) black holes in the centers of some galaxies, and tellar-mass black holes are the end points galaxy, under the assumption that all stars of ini- that from studying BHXRBs on humanly accessi- of the evolution of the most massive stars. tial mass >10 times that of the Sun met this fate. ble time scales, we may be learning about the forces SThecollapseofanironcoreof>3solar The strongest evidence for the existence of this that shaped the growth of galaxies over the life- masses (M⊙) cannot be stopped by either electron population of stellar-mass black holes comes from time of the universe. Between the stellar-mass black or neutron degeneracy pressure (which would other- observations of x-ray binary systems (XRBs). In holes and the supermassive, there could be a popu- Downloaded from wise result in a white dwarf, or neutron star, respec- XRBs, matter is accreted (gravitationally captured lation of intermediate-mass black holes (IMBHs), 2 5 tively). Within the framework of classical general into/onto the accretor), releasing large amounts of with masses in the range of 10 to 10 M⊙.These relativity (GR), the core collapses to a singularity gravitational potential energy in the process. The may be related to the ultra-luminous x-ray sources that is cloaked in an event horizon before it can be efficiency of this process in releasing the gravita- (ULXs), very luminous x-ray sources that have been viewed. Like a giant elementary particle, the result- tional potential energy is determined by the ratio observed in external galaxies. However, the problem ing black hole is then entirely described by three of mass to radius of the accretor. For neutron of the nature of these sources is still unsettled, and parameters: mass, spin, and charge (1). Because stars, more than 10% of the rest mass energy can alternative options involving stellar-mass black galaxies are old—the Milky Way is at least 13 bil- be released—a process more efficient at energy re- holes are still open. lion years old—and the most massive stars evolve lease than nuclear fusion. For black holes, the ef- quickly (within millions of years or less), there ficiency can be even higher (3),butthepresenceof Black Hole X-ray Binaries are likely to be a large number of such stellar-mass an event horizon—from within which no signals can There are several different approaches to clas- black holes in our galaxy alone. Shapiro and ever be observed in the outside universe—means sifying BHXRBs and their behavior, each of Teukolsky (2) calculated that there were likely to that this accretion power may be lost. which can lead to different physical insights. One be as many as 108 stellar-mass black holes in our In some of these systems, dynamical mea- important approach is to look at the orbital pa- surements of the orbit indicate massive (>3 M⊙) rameters, and the most important of these is the accretors that, independently, show no evidence mass of the donor star because it relates to the 1 Physics and Astronomy, University of Southampton, Southampton for any emission from a solid surface. The first age of the binary. High-mass x-ray binaries have SO17 1BJ, UK. 2Istituto Nazionale di Astrofisica–Osservatorio Astronomico di Brera, Via Emilio Bianchi 46, I-23807 Merate such candidate black hole x-ray binary (BHXRB) OB-type (5) massive donors and are young sys- (LC), Italy. system detected was Cygnus X-1, which led to a tems, typically with ages less than a million years *To whom correspondence should be addressed. E-mail: bet between Kip Thorne and Stephen Hawking or so. They are clustered close to the midplane of [email protected] as to the nature of the accreting object (although the Galactic disc and associated with star-forming
540 3 AUGUST 2012 VOL 337 SCIENCE www.sciencemag.org SPECIALSECTION regions. Cygnus X-1 is one of only a small num- could be applied to other systems (8, 9). In the Compton scattered by the corona). X-ray spec- ber of high-mass BHXRB systems. following sections, we describe the evolution of troscopy also often reveals strong iron emission Most BHXRBs are, however, low-mass x-ray the outburst through Fig. 2, top; a sketch of likely lines (which can be fluorescence lines from neutral binaries, which are almost certainly much older geometries in the soft, intermediate/flaring, and or ionized iron) in this phase. This iron line can than the high-mass systems and as a result have a hard states is presented in Fig. 2, bottom. An ani- often be fit by a relativistically broadened model, larger scale-height distribution in the galaxy and mation of an outburst in the hardness-intensity implying an origin very close to the black hole, and are hard to associate with their birth sites. In these diagram (HID) is presented in the supplementary can in turn be used to estimate the spin of the black systems, the companion donor star is of lower mass materials (movie S1). hole. This is because of the innermost stable cir- (typically less than 1 M⊙) than the black hole. The rising phase of the outburst (A → B). cular orbit (ISCO): Within this radius, matter can We now know of a sufficiently high number of Sources in quiescence are rarely regularly moni- no longer follow a circular orbit and will cross the accreting x-ray binaries to be able to study their tored, and so usually the first thing we know about black hole event horizon on very short time scales galactic population, which we will not cover here. an outburst is an x-ray source rising rapidly in (milliseconds for a black hole of a few solar masses). Most of these low-mass BHXRBs undergo tran- luminosity, as detected by x-ray all-sky monitors. The size of the ISCO depends on the spin of the sient outbursts, in that they typically black hole, ranging from 6 RG (10) spend most of their time in faint qui- for a nonrotating (Schwarzschild) escent states before going into bright black hole to 1 RG for a maximally outbursts that approach the Eddington Jet rotating (maximal Kerr) black hole. (6) limit and last from months to X-ray Accurate measurements of the de- years. It is the detailed study of these heating Companion gree of gravitational redshift affect- star outbursts, when the mass accretion Disc wind ing the line can be used to infer how rate onto the central black hole can Stream-impact close the line is to the black hole, and change by orders of magnitude in just point from this the spin of the black hole days, that has allowed us to make itself, although both observation and dramatic strides in our overall under- modeling are complex. During the standing of black hole accretion. hard state, characteristic time scales of on March 28, 2013 The outbursts of low-mass sys- variability, called quasi-periodic oscilla- temsarelikelytobetriggeredbya Accretion tions (QPOs), are also seen to decrease, hydrogen ionization instability in the stream which may correspond to changing accretion disc while the mass trans- viscosity or decreasing characteristic fer rate from the donor star remains radii in an evolving accretion disc. steady. Initially, the mass transfer rate In this state, sources are always ob- is greater than the rate of accretion served to also show relatively steady onto the central black hole. The disc, Accretion radioemissionatgigahertzradiofre- disc initially neutral, slowly rises in temper- quencies (11). This radio emission www.sciencemag.org ature as the mass accumulates, until (LR) correlates in strength with the at some point it reaches ~4000 K and x-rayemission(LX) in a nonlinear ’ b the hydrogen starts to become ion- Fig. 1. An artist s impression of a low-mass BHXRB. The major components way: LX º LR ,where0.6
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During this phase, the behavior of SOFT X-ray spectrum HARD hard transition, although also show- the jet, revealed by infrared and radio ing a range of luminosities at which it observations, also begins to change. C B can occur (even in the same source), The infrared emission drops almost D generally occurs at a luminosity of a as soon as the state transition begins few percent of the Eddington lumi- (14), indicating a change in the jet nosity (24). In fact, the soft state has properties (density and magnetic field) never been convincingly observed close to the black hole. in any BHXRB at luminosities be- The radio emission begins to vary low 1% Eddington. By the time the more dramatically, showing oscilla- source reaches the canonical hard tions and flare events superposed on state again, with almost exactly the an overall decline (8, 15). At a cer- same spectral and timing character- tain point, there are one or more large istics as the initial hard state, the jet E radio flares, which can be two or more has reappeared, and the accretion
X-ray luminosity F orders of magnitude more luminous disc wind is gone. Once in the hard than the previous existing, steadier jet state, the source decline continues, in the hard state. In several notable typically below the detection levels cases, high-resolution radio observa- of all-sky or regular x-ray monitoring, tions after such flares have directly and are observed only occasionally resolved radio- or even x-ray–emitting A until their next outburst. These quiet blobs moving away from the central phases are not without interest, how- black hole (16, 17), which can be ever, for it is during these periods kinematically traced back to the time that—without the glare of the bright of the state transition. It has been re- accretion disc—researchers are able cently shown that in some cases, the to accurately measure the orbital mo- on March 28, 2013 ejection is coincident in time with the tions of the companion star using appearance of the strong QPOs (15). optical telescopes and hence esti- The soft state (D → E). As the mate the mass of the black hole it- spectral transition continues, these self (25, 26). strong QPOs disappear, and the over- These cycles of activity reveal all level of x-ray variability drops as clear changes in the way that an ac- the soft state is entered. The radio creting black hole responds to an in- emission also fades away in most Fig. 2. (Top) The HID. The horizontal axis represents the “hardness” or color of the crease in accretion rate, varying the
cases, probably indicating a cessa- x-ray emission from the system, which is a crude but effective measure of the x-ray form and degree of both the radiative www.sciencemag.org tion of core jet activity (18, 19). spectrum. The vertical axis represents the x-ray luminosity. By analogy with the and kinetic ( jets and winds) outputs The x-ray spectrum is now dom- Hertzsprung-Russell diagram, in which the lifetime of a star can be tracked, in the from the liberated gravitational poten- inated by the accretion disc compo- HID the evolution of a black hole outburst can be tracked. Each point corresponds to tial energy. These patterns have been nent, although there is a weak (a few a single observation. Observations on the righthandsideoftheHIDareconsidered observed more than 30 times with “ ” “ ” percent of the total luminosity) high- to be in hard x-ray states, and those on the left are in soft states. Although the very few exceptions, and no system energy tail (probably of nonthermal detailed patterns of individual outbursts (including in some cases multiple outbursts that strongly contradicts the estab- origin) to the spectrum that extends from the same source) differ, one of the major steps of the past decade was the lished empirical pattern of behavior to megaelectron volt energies. The realization that the overall picture outlined below turns out to be applicable to has ever been observed. These studies Downloaded from essentially all BHXRB outbursts. (Bottom) Illustrations of likely geometries in the soft state is generally the longest last- provide us with hope of being able soft, intermediate/radio flaring, and hard states (from left to right). ing phase of an outburst, often per- to estimate the radiative and kinetic sisting for a weeks at a more or less output at any given phase of, and constant luminosity before beginning a slow de- The latest development in understanding the cumulatively for, the outburst of a BHXRB. We cline. This marks the point at which the accretion overall geometry and energy budget of the differ- may then use that to understand how such feed- rate onto the central accretor starts to drop. In the ent states has been the realization that these soft back affects the local ambient media, energizing hardness-luminosity diagram (HLD) the source still states also ubiquitously produce a strong accretion and accelerating particles and seeding magnetic makes some rapid changes in x-ray color, includ- disc wind that is focused in the plane of the accre- field. We can seek to calculate how the cumulative ing occasional rapid excursions to hard x-ray states tion disc and may carry away a large amount of loss of angular momentum in winds and jets may (20) that can be associated with brief reactivation mass from the accretion flow (22, 23). Previously, affect the evolution of the binaries. If we can truly of the jet as observed in the radio band (19). Dur- kinetic outflow was only well established in the tie properties of the accretion flow to the black hole ing the luminosity decline in the soft state, mea- hard state, which shows the strong radio jet, but spin, we may learn about the evolution of black surements of the luminosity (L) and temperature we now know that both states have outflows, holes and understand how and when to make (T) of the accretion disc component follow a rela- albeit of rather different natures. observations to test GR in the strong field limit. tion close to L º T 4, as expected for a blackbody The return to the hard state and quiescence Theimportanceoftheseresultswouldbeeven of constantly emitting area. This is interpreted as (E → F). Eventually, as the central mass accre- greater if it could be demonstrated that they also indicating that the accretion disc has a constant tion rate continues to fall the BHXRB makes a shed light on accretion and feedback in the most radius and structure, which may correspond to transition back from the soft to the hard state. In massive black holes, those in active galactic nu- the ISCO and hence can be used—as with the iron nearly all cases, this soft → hard transition oc- clei (AGNs), which may have helped to stall cool- emission line in bright hard states—to estimate curs at a lower luminosity than that of the earlier ing flows in galaxy clusters, shaping the growth of the black hole’sspin(21). hard → soft transition. Furthermore, the soft → galaxies—still one of the biggest problems in
542 3 AUGUST 2012 VOL 337 SCIENCE www.sciencemag.org SPECIALSECTION extragalactic astrophysics (27). There are sugges- few cases, most notably of the bright system X-1 in sequent study (40) has reported a correlation be- tions that this may well in fact be the case. Com- the galaxy M 82. In order to scale the observed tween transient jets and spin measurements derived parison of x-ray, optical, and radio surveys of frequencies (in the 10 mHz range) to those from from accretion disc modeling in the soft state. A AGNs may show similar patterns of coupling be- bona fide stellar-mass objects, other parameters key source in this story turns out to be, once again, tween accretion and jets (28). More quantitative- must be determined, such as the type of QPO and Cygnus X-1: Recently, both disc and iron line ly, although not without its sceptics, it has been the association to spectral features, all of which modeling have converged on a very high spin for shown that all accreting black holes follow an makes this method still inconclusive. the black hole in this system (41, 42), which is also empirical relation between their mass and x-ray The amount of observational data on ULXs nearby with a very accurately measured distance and radio luminosities (29, 30). and HLXs is still much less rich than that of (43) and has a well-studied and powerful jet (44), stellar-mass systems, and extensive observational although as yet no major ejection events. Ultra Luminous X-ray Sources campaigns are needed in order to reach a better A long standing question for black hole accre- Modern x-ray observatories are easily capable of understanding of these objects and their nature. tion that remains unanswered is how much of the detecting the brightest x-ray sources in external gal- accreting matter and gravitational potential ener- axies. Of these, we often observe a high luminosity Conclusions and Open Issues gy actually crosses an event horizon. Models in tail of the population at or above the Eddington Observational perspective. The revolution in our which liberated potential energy may be trapped limit for a stellar-mass black hole. If these ULXs understanding of black hole accretion that has taken in the flow and unable to escape before it crosses contain stellar-mass black holes, then in order not place over the past decade has been due in large the event horizon appear to be able to explain why to violate the Eddington limit their emission must be part to the extremely good coverage of black hole some black holes are so faint when in quiescence highly nonisotropic (beamed), or the most luminous binary outbursts by x-ray observatories. We single (45), although it turns out that such systems are examples must contain a black hole of much higher out the Rossi X-Ray Timing Explorer (RXTE), also almost universally associated with a powerful mass, 100 to 1000 times the mass of the Sun. A third whose flexible scheduling and high time-resolution jet, which could be an alternative sink for “missing” option is that these objects do indeed emit well data led to the model presented in Fig. 2. What was radiative output (46). The accretion disc wind in above the Eddington limit, which naturally would clearly missing were comparably high-cadence ra- soft states also throws up some interesting questions. pose theoretical problems. It is most probable that dio and infrared observations to track the jet as well Is the wind responsible for halting the jet during the observed sample is not homogeneous and in- as the accretion flow. RXTE was decommissioned the state transition, or are they both symptoms of on March 28, 2013 stead composed of more than one class (31). For the in January 2012, just at the time when new radio something deeper? Does the existence of kinetic few most luminous, called hyper-luminous x-ray arrays—with improved sensitivities and fields of view outflow in both soft and hard states demonstrate sources (HLXs), the interpretation in terms of a and whose key science goals embraced the astro- that such outflows, carrying away mass, energy, and stellar-mass black hole appear to be difficult to re- physics of variable and transient sources—were angular momentum, are a necessary component for concile with the observations. The most luminous being constructed or commissioned. In the coming accretion? These jets should be considered in the HLX, in the galaxy ESO 243-49 (32), emits a factor decade, the Square Kilometer Array precursors broader context of accreting objects, such as young of 1000 higher than the Eddington limit and shows MeerKAT and Australian Square Kilometre Ar- stellar objects, cataclysmic variables, gamma ray discrete radio flaring events (33), leaving the op- ray Pathfinder (ASKAP) will provide orders-of- bursts (GRBs), unusual supernovae and—most
4 www.sciencemag.org tion of a 10 M⊙ black hole emitting close to the magnitude more radio coverage than previously closely related—neutronstarXRBs,allofwhich Eddington limit as the most probable. For fainter has been possible (37), but RXTE will not be there also show jets and winds. Of these, we might ULXs, the discussion is still open. to provide the x-ray data, nor is there a clear, pub- hope that in the future we can link long-duration A number of ULXs are associated with emission- licly orientated replacement in the works. It can GRBs—which may correspond to a short phase of line nebulae. Because these nebulae are ionized by only be hoped that a new mission, with com- accretion at very high rates onto a newly created the radiation from the ULX, they can be used as a parable timing and all-sky monitoring capabilities, black hole—to the empirical models presented bolometer to estimate its total luminosity (34). From will be available in the coming decade to work here. The current hot topic of possible tidal dis- such studies, it emerges that their radiation is at with these new radio arrays. ruption events and their consequent rapid-change most mildly beamed. Other observational evidence, Key unanswered questions. The study of black burst of accretion onto a supermassive black Downloaded from such as observations of super-Eddington episodes hole accretion allows us to probe both the details of hole, with associated jet formation (47, 48), may from sources in the Milky Way and the discovery GR in the strong field regime (38) and to under- also be possible to integrate into this broader of a population of ULX associated with star-forming stand the role of black holes in the conversion of model. Much remains to be done. regions, have led to the suggestion that ULXs are gravitational potential energy into kinetic energy a phase of the evolution of high-mass x-ray bi- and radiation. This feedback of energy from these References and Notes 1. Charge is assumed to be unimportant on macroscopic naries. In the absence of a direct mass measurement cosmic batteries is important on all scales: from scales in astrophysics because the Coulomb force is much for the compact object such as those obtained from heating the local interstellar medium in our own larger than the gravitational force, causing any charge optical observations of galactic systems, which are galaxy, to affecting the growth of the largest gal- separation to be rapidly neutralized. difficult to obtain with current optical telescopes, axies and the heating of cooling flows in the centres 2. S. L. Shapiro, S. A. Teukolsky, Black Holes, White Dwarfs much of the observational effort is aimed at find- of galaxy clusters. To this end, understanding in and Neutron Stars (Wiley, New York, 1983). — 3. Because the size of the black hole event horizon varies ing features that can be scaled to BHXRBs in our more detail the relativistic jet how it forms, and linearly with mass, the ratio of mass to radius, and hence own galaxy. Different states and state transitions how much power it carries—is a key question. It accretion efficiency, is the same (ignoring spin for now) for have been reported for several ULXs (35). has long been suggested that relativistic jets could black holes of all masses. The measurement of accretion disk parameters be powered not by the accretion flow but by the 4. J. E. McClintock, R. A. Remillard, in Compact stellar X-ray sources, W. Lewin, Michiel van der Klis, Eds. (Cambridge (36) are one way to obtain an indication of the mass spin of the black hole (which can contain a vast Astrophysics Series, no. 39, Cambridge Univ. Press, of the object through the inner radius of the ac- amount of energy). X-ray spectral observations can Cambridge, 2009), pp. 157–213. cretion disk, which, if located at the ISCO, scales allow us to estimate the black hole spin, and we 5. OB-type stars are the hottest and most luminous, with the linearly with the mass of the black hole. The de- may compare this with estimates of the jet power. shortest lives. 6. The Eddington luminosity is that luminosity at which, under tails of the spectral model are very complex, but It is currently unclear where this leads us: One the simplifying assumptions of pure hydrogen accretion very large differences can be trusted. On the side study (39) found no correlation between jet power and spherical symmetry, the outwards radiation force (on the of time variability, QPOs have been detected in a and reported spin measurements, whereas a sub- electrons) balances the inwards gravitational force (on the
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REVIEW eventually condensed to form stars and the luminous portion of galaxies. The first halos and galaxies that on March 28, 2013 form are small, no more than about 1 million times The Formation and Evolution the mass of our Sun, and present-day galaxies, of up to hundreds of billions of M⊙, have been assem- of Massive Black Holes bled, bottom-up, from these smaller building blocks. The first MBHs must have formed from within M. Volonteri1,2 these first proto-galaxies and then grown with them. One of the most popular theoretical scenarios The past 10 years have witnessed a change of perspective in the way astrophysicists think about massive black (Fig. 1) associates the first MBHs with the rem-
holes (MBHs), which are now considered to have a major role in the evolution of galaxies. This appreciation nants of the first generation of stars (7), formed out www.sciencemag.org was driven by the realization that black holes of millions of solar masses and above reside in the center of of pristine gas, which did not contain heavy ele- most galaxies, including the Milky Way. MBHs also powered active galactic nuclei known to exist just a few ments yet (8). Simulations of the formation of stars hundred million years after the Big Bang. Here, I summarize the current ideas on the evolution of MBHs through in proto-galaxies (9) suggested that the first genera- cosmic history, from their formation about 13 billion years ago to their growth within their host galaxies. tion of stars might have contained many stars with masses above a few hundred M⊙.Thisisbecause hen astronomers refer to black holes, their host galaxies, and typically only one hole is of the slow subsonic contraction of the gas cloud— two different flavors exist. We know of observed per galaxy (5). We know how stellar black a regime set up by the main gas coolant, molecular Wstellar black holes, with masses up to a holes form: They are the remnants of massive stars, hydrogen, which is much more inefficient than the Downloaded from few tens times the mass of our Sun (M⊙)(1), and roughly 10 M⊙ and above (6). Yet, how MBHs form atomic line and dust cooling that takes over when massive black holes (MBHs), with masses up to and evolve inside galaxies is one of the most fas- heavy elements are present. If stars more massive billions of times that of the Sun, which are the focus cinating mysteries in modern astrophysics and one than roughly 250 M⊙ form, no process can produce of this Review. Most of the best-studied MBHs that astrophysicists seek to unravel through theoret- enough energy to reverse the collapse. Thus, a MBH have masses in the range of tens of millions to a few ical and observational work. of ~100 M⊙ is born. Whether most of the first stars billion M⊙ (2, 3). The MBH population may ex- were born with such large masses is still an open tend down to smaller masses, though this range is How Do Massive Black Holes Form? question, and recent simulations revise the initial much harder to probe. The record for the smallest MBHs must have formed from the same material estimates of the stellar masses to possibly much MBH currently belongs to the dwarf galaxy NGC from which galaxies and the rest of the universe is lower values, just a few tens of solar masses (10). If 4395, which is thought to contain a black hole weigh- composed. Stars and gas represent the baryonic this is the case, it is unlikely that the first stars have ing only few hundred thousand M⊙ (4). Observa- content of galaxies, in contrast to nonbaryonic dark generated the first MBHs. A 10 M⊙ black hole would tionally, there seems to be a gap between the two matter that does not interact electromagnetically, have a very hard time growing by several billion types of black holes, which scientists take as a hint but only gravitationally with its environment. In the M⊙ to explain the observed population of MBHs. that there are native differences between stellar black standard picture, the mass content of the universe is MBHs with substantial initial masses, thousands holes and MBHs. In brief, stellar black holes are dominatedbycolddarkmatter,withbaryonscon- to millions of M⊙, can form as a consequence of dy- scattered in large numbers throughout galaxies, tributing up to a 15% level. Starting from small namical instabilities that involve either the gaseous whereas MBHs tend to be located at the center of density fluctuations in a quasi-homogeneous uni- or stellar content of proto-galaxies. In proto-galaxies, verse, dark matter perturbations grew under the the gaseous component can cool and contract until 1Institut d’AstrophysiquedeParis,Paris,France.2University of effect of gravity to the point that they disconnected rotational support takes over: Centrifugal sup- Michigan,DepartmentofAstronomy,AnnArbor,MI,USA.E-mail: from the global expansion of the universe, became port typically halts collapse before densities required [email protected] self-gravitating, and formed halos within which gas for MBH formation are reached. Gravitational
544 3 AUGUST 2012 VOL 337 SCIENCE www.sciencemag.org