Black Holes - Sky & Telescope
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Black Holesfront cover and above : mark garlickBlack holes capture the imagination. These rents in the fabric of spacetime let nothing, not even light, escape. Yetwhen they feast, supermassive black holes nesting in galaxy centers become powerful beacons. As gas flows intotheir gaping maws, it heats up, glowing brightly enough to be seen from the early universe.Four articles from past issues of Sky & Telescope reveal the mysteries of supermassive black holes, how they form,and the role they play in galaxy formation. One day, we might even be able to observe the beasts themselves. 1A Quasar in Every Galaxy?Observations and theory show that nearly all major galaxies had blazing cores in the past— but only for brief, glorious intervals.By Robert Irion8How Black Holes Helped Build the UniverseWithout black holes, the universe around us would be unrecognizable, and we might not even exist.By Christopher Wanjek14Spinning Hearts of DarknessAstronomers are measuring the rotation rates of black holes to determine their formation and history.By Laura Brenneman20Einstein’s ShadowA planet-wide telescope sets its sights on the well-kept secrets of black holes.By Camille Carlisle
Observations and theory tell astronomers thatnearly all major galaxies had blazing cores inthe past — but only for brief, glorious intervals.Centaurus A (NGC 5128), seen in thiscomposite X-ray, visible-light, and radioimage, is nowhere near as brilliant as aquasar, but it is the closest active galaxy to Earth. This large elliptical galaxyrecently swallowed a smaller spiral,whose remains appear as dark dustlanes. The merger funneled huge quantities of gas into Centaurus A’s core,where it feeds a supermassive blackhole. As the black hole accretes the gas,some of the material is blasted out ofthe galaxy in powerful outflows (green).NASA / CXC / M. Karovska / NRAO / VLA / J. Van Gorkom / J. Condon / Digitized Sky Survey UK Schmidt Image / STScI & others1 July 2006 Sky & Telescope
?AQuasarinEveryGalaxyBy Robert IrionBut the steady glow of galaxies doesn’t tell the wholestory. Several lines of research now suggest that nearlyall mature star cities threw tantrums in their youth.At least once in every major galaxy’s history, radiationblazed from hot matter accreting violently onto a supermassive black hole in the core. Astronomers thinkthese quasar-like outbursts were relatively brief. Evenso, they determined the fates of the graceful galaxieswe see today.Theorists and observers reached this conclusionby focusing on the early epochs when ravenous blackholes gobbled most of their mass. Ironically theseblack holes limited their own growth by unleashingtorrents of energy that drove away the surrounding gas(S&T: April 2005, page 42). These waves of unrest alsodictated the ebb and flow of starbirth in the host galaxies. This feedback process forged a close link betweenmassive black holes and their surrounding stars.This view of abrupt but dazzling mayhem in majorgalaxies represents a profound conceptual shift in ourunderstanding of cosmic evolution. Long ago astronomers grew accustomed to a universe with extremeranges of behaviors. Stately spirals like the Milky Wayand Andromeda seemed normal; quasars and othertypes of active galaxies, such as blazars and Seyfert galaxies, were the freaks. But researchers now regard thefreaks as unusual only in the time domain. It’s all a mat-NASA / Hubble Heritage Team / STSCI / AURAfor every superstar in the Hollywood firmament, thousands of pedestrian actors just try to make ends meet. Likewise, for every brilliantquasar that illuminates the universe, countless other galaxies — like ourMilky Way — shine with the unspectacular light of middle age.M87 is an enormous elliptical galaxy that containsa 3-billion-solar-mass black hole in its core. Thismonster powers a jet, seen in this Hubble image,which stretches across thousands of light-years.ter of when in their evolution we happen to see them.“Quasars appear rare because they are suicidal,”says Abraham Loeb (Harvard-Smithsonian Center forAstrophysics). “They accrete a lot of gas, then theyshut themselves off. On cosmological time scales, it’sjust like an explosion.” The Milky Way’s core certainlyerupted in this way before settling down. But even atits peak vigor, our galaxy didn’t hold a candle to thequasars raging in the far reaches of the universe.Sky & Telescope July 2006 2
pa quasar in every galaxy?Gary Bower / Richard Green / STIS Instrument Definition Team / NASA (2)3 July 2006 Sky & Telescope1010M87109Black hole mass (Suns)Centaurus Asource: Laura Ferrarese / David Merrit tBlack Holes EverywhereWith luminosities that often exceed that of the billionsof stars in their host galaxies, quasars are the most energetic members of a class known as active galactic nuclei,or AGN. Quasars can shine so brightly only by devouringhuge quantities of gas — enough to build black holes packing hundreds of millions to billions of solar masses. OurMilky Way, by contrast, hosts a modest central black holecontaining “only” about 4 million solar masses.Direct observational evidence of these monster blackholes first surfaced in the 1990s, when the Hubble SpaceTelescope probed the cores of 40 nearby galaxies. Hubble’sspectrograph revealed pronounced Doppler shifts, clearsigns that stars at the very center of almost every observedgalaxy whirl at breakneck speeds toward us on one sideand away from us on the other. The rapid stellar orbitsindicated deep gravitational fields carved by supermassiveblack holes.The results of Hubble’s survey surprised even the observing team. “At the beginning I thought black holes wererare, maybe 1 galaxy in 10 or 100,” says Douglas Richstone(University of Michigan, Ann Arbor). “Now we’ve shownthey are standard equipment.”There is one key caveat, however. Only galaxies witha central bulge host a monster black hole. Bulges are thenearly spherical swarms of old stars that surround thecores of most large galaxies. But as many as 15% of spiralgalaxies consist only of starry disks, with no bulge whatsoever. The face-on spiral M33 in Triangulum is a nearbyexample. “It has no bulge and no black hole that we candetect,” says Richstone.Still, most large galaxies follow a remarkable trend. In2000 teams led by Karl Gebhardt (University of Texas,Austin) and Laura Ferrarese (Rutgers University) showedthat the masses of black holes inside galaxy cores correlatevery strongly with the velocities of stars orbiting throughtheir host bulges. A higher bulge mass produces faster stel-108M81107M32106Milky Way105NGC 205104M33103204060100200400 600Stellar velocities (kilometers per second)lar orbits, on average. And the researchers found that thehigher the bulge mass, the higher the mass of the centralblack hole.This landmark discovery, called the M-sigma relation,* catalyzed the field. It provided compelling evidence that central black holes are intimately connected to the evolutionof their host galaxies. Most astronomers had suspected thatgalaxies and their black holes grew simultaneously duringperiods when they accreted lots of gas, possibly during galaxy mergers. “To me, the M-sigma relation made the physi* M is the mass of the central black hole, and sigma (σ) is the velocity dispersion of the bulge, a measure of how fast its stars are moving.The Hubble Space Telescope imaged the coreof M84, a giant ellipticalgalaxy in the Virgo Cluster. The orange rectangleindicates the nuclear region observed by Hubble’simaging spectrograph,which measured the velocities of gas near M84’scenter and found a zigzagpattern (inset) indicatingextremely fast orbital motions. The gas orbits atspeeds of 400 kilometersper second within 26 lightyears of the center, indicating an invisible massequal to 300 million Suns.
NASA / CXC / PSU / David Alexander & othersActive Galaxies Near and FarTo understand the feedback processes at work in galaxies,astronomers use deep surveys at various wavelengths totrace black-hole activity across billions of years. In visiblelight the ongoing Sloan Digital Sky Survey has played a pivotal role. Its 3.5-meter (140-inch) telescope in New Mexicohas detected about 70,000 quasars. Each quasar’s spectrumreveals how much its light has been redshifted by cosmicexpansion, which astronomers convert into the age whenthe quasar was active.Some of the most powerful known quasars arose earlierthan a redshift of 6, which translates to 1 billion years afterthe Big Bang. The most distant quasar, at redshift 6.42,lived at a cosmic age of only 870 million years. “These areamong the most luminous objects the universe has everseen,” says Xiaohui Fan (University of Arizona, Tucson).Theorists think these blazing objects arose in special placeswhere matter concentrated most densely in the wake of theBig Bang. Seed black holes of a few thousand solar masses— created by an as-yet unknown process — accumulatedthose titanic masses by devouring gas and other blackholes. But those intensely luminous quasars were rare. Sofar Sloan has imaged just 19 such objects with redshifts of6 or higher.In a universe of hundreds of billions of galaxies, the70,000 Sloan quasars seem like a drop in the bucket. Fortunately, other eyes on the sky are finding more. Quasarsspew copious X-rays as million-degree gas spirals towardthe supermassive black hole. NASA’s Chandra X-ray Observatory has exposed those fiery pinpricks with a narrow,deep survey in each hemisphere. Whereas the Sloan survey sees about 10 quasars per square degree, Chandra’sexposures reveal 7,000 active galaxies in the same area. “Athigh redshifts we find active galaxies up to 100 times lessluminous than the rare powerful Sloan quasars,” says NielBrandt (Penn State University). “These are the typical activegalactic nuclei in the universe.”When Brandt extrapolates Chandra’s detections to therest of the sky, he calculates that the observatory can seethe cores of 5% of all “decent-size” galaxies in the universe.That’s a plausible statistic, says Brandt, because astronomers think the cores of major galaxies light up as X-raybright AGN just a small fraction of the time. Moreover,Chandra may miss galaxies sochoked by thick clouds of gasand dust that X-rays cannotLeft: A 23-day Chandra deep exposureescape efficiently.in Ursa Major reveals hundreds of pointFortunately, NASA’s infrasources in an area about 60% the sizered Spitzer Space Telescope,of the full Moon. Most are very distantlaunched in August 2003,supermassive black holes gobbling upcan detect some of these hidenormous quantities of gas and shiningden AGN. Infrared light canas active galactic nuclei. Center: Withstream through the denseits ability to peer through dust, NASA’storuses thought to shroudinfrared Spitzer Space Telescope has resome of the most vigorouslyvealed thousands of active galactic nuaccreting galaxy cores. Inclei previously hidden by gas and dust,2005 two teams of researchsuch as the quasar in Draco shown iners reported that Spitzer findsthis composite Spitzer and radio image.the glowing cores of activeRight: The Sloan Digital Sky Survey capgalaxies everywhere it looks,tured this quasar with a redshift of 6.4(arrowed) in Ursa Major.SDSS Collaboration / Xiaohui Fancal connection clear,” saysMegan Urry (Yale University).“All galaxies with supermassive black holes must havegone through this activephase.”The tightness of theM-sigma relation amazedtheorists as well. “It’s almostmagical, because the scalesare so completely different,”says Nickolay Gnedin (University of Colorado, Boulder).“If you draw a galaxy on asheet of paper, the centralblack hole would be the sizeof an atom. It’s hard to understand how the black holesknow about the galaxies, orhow the galaxies know aboutthem.” But observers andtheorists are converging onan explanation.NASA / JPL / Caltech / Alejo Martínez SansigreUsing Hubble and otherinstruments, astronomerssurveying dozens of galaxieshave found a remarkablefact: The mass of the centralsupermassive black holecorrelates with overall stellar velocities in the galaxy’sspheroidal component (either an entire elliptical galaxy or the central bulge of aspiral galaxy). These velocities, in turn, correlate withthe mass of the spheroidalcomponent. The discovery ofthis “M-sigma relation” toldastronomers that the evolution of black holes and theirhost galaxies is intimatelyconnected.Sky & Telescope July 2006 4
pa quasar in every galaxy?Source: Günther Hasinger (MPI) & othersRelative density ofactive galactic nucleiespecially between redshifts of 2 and 1 (roughly 3Time since Big Bang (billions of years)billion to 6 billion years after the Big Bang) — theepoch when most galaxies assembled. “Most quasars13.76.03.42.21.61.21,000at these redshifts and beyond are hidden behind gasand dust,” says Urry. “That makes sense. As those galaxies are collapsing, they should be in the messiest,100dirtiest environments.”One recent study combined optical and X-ray observations of dusty galaxies and their central black10holes. David Alexander (Cambridge University, England) and his colleagues examined 20 “submillimeter”1galaxies around redshift 2. These luminous galaxies012345were originally spotted by the 15-meter James ClerkRedshiftMaxwell Telescope at Mauna Kea, Hawaii, which canThis graph shows the relative cosmic density of active galactic nucleidetect submillimeter waves radiating from dusty stel(those with average luminosities) by redshift. Quasars were much morelar nurseries. An optical study led by Scott Chapmancommon in the distant past, meaning that supermassive black holes ac(Caltech) showed that these galaxies create about onecumulated most of their mass long ago and have largely settled down tonew star every day — 100 times higher than the Milkya state of quiescence.Way’s rate. Chandra X-ray data revealed that 15 of the20 submillimeter galaxies have black holes activelyfeeding on gas.galaxies do the trick, a view supported by a model created“These galaxies are forming a lot of stars, and at the sameby Philip Hopkins and Lars Hernquist (both at the Harvardtime you have almost continual black-hole growth,” saysSmithsonian Center for Astrophysics) and their coworkers.Alexander. “We’re seeing the construction of massive galaxTelescopes reveal galaxy mergers everywhere, but theyies and their central black holes.” The team claims that thiswere far more common in the early universe, when galaxis the first solid evidence of how the mysterious M-sigmaies were closer together. To find out how these crashesrelation arose. Alexander thinks the submillimeter galaxieseventually became titans like M87 — a nearby giant elliptical affected the black holes and the internal dynamics of galaxies, the team created computer simulations that varied thegalaxy in the Virgo Cluster whose 3-billion-solar-mass blackgalaxy masses, gas contents, and collision angles.hole propels a jet to near-light speed.The results, published in March 2006, produced a newinsight. “The evolution of quasars is more complicatedModels of a Violent Universethan people assumed,” says Hernquist. “Their activity isThis flurry of multiwavelength observations gave theoristswhat they needed to construct more accurate models of gal- sporadic, and they are visible as intense sources only forvery short periods.” The simulations suggest that a blackaxy growth. By mid-2005 thehole accumulates most of its mass in extreme feedingconsensus was clear: Massiveepisodes — triggered by mergers — that switch on a brightblack holes control the evoluThis supercomputer simulation depictsquasar for just 1% of a galaxy’s existence. For the rest of itstion of every major galaxy.two spiral galaxies merging to formlifetime, the nucleus is mostly dormant.Black holes can’t do it inan elliptical. Each frame is about 300Observations support this scenario. A team led byisolation, however. A lonemillion light-years across. The numberCharles Steidel (Caltech) reports that ordinary galaxies atgalaxy with a black hole atin each frame represents the amountredshifts of 2 to 3.5 are 50 times more common than quaits heart will happily swirlof cosmic time (in billions of years)sars during that era — just as one would expect for a popufor eons, and the hole willsince the simulation began. When twolation of mostly quiescent black holes.eat only what happens togalaxies collide, their central blackIn this new picture, black holes and galaxy bulges formdrift nearby. To trigger rabidholes accrete huge quantities of gastogether in a sudden, violent “blowout phase” triggeredgrowth spurts, a black holeand eventually merge (at a time of 1.48by a merger. The impact on the surrounding galaxy isneeds a major disturbance tobillion years as simulated here). Theprofound. Energy released by matter plunging toward thefunnel torrents of fresh gasgalaxy thus shines brightly as a quasarblack hole ignites the quasar and heats gas throughoutinto the core.for a brief period of time. In the fourththe galaxy’s core. But the gas can only take so much heatCollisions between gas-richframe, the black holes are blowing outlarge amounts of gas.0.210.500.861.211.48Philip Hopkins / Lars Hernquist / Volker Springel (6)5 July 2006 Sky & Telescope1.93
UPPER LEFT: Two large spiral galaxiesapproach each other before merging.s&t: gregg dindermanUPPER RIGHT: The supermassive blackholes in each galaxy eventually merge.Large amounts of gas are funneled intothe newly created supermassive blackhole. As the matter falls into the abyss,it radiates huge amounts of energy, andthe galaxy thus shines as a quasar orother type of active galaxy.LOWER RIGHT: The supermassive blackhole in the newly formed mega-galaxygenerates huge outflows of gas, whichprevent further gas from falling into thegalaxy’s central regions, shutting offthe quasar. Lacking gas, the galaxy willevolve into a “red and dead” ellipticalwith virtually no star formation.before it all escapes, like water from a boiling pot. Biggergalaxies have enough gravity to hold onto hotter gas, so theblack hole grows even more massive.Astrophysicists think this explains why small galaxybulges host small black holes, while big bulges host bigblack holes. “There is a critical point in the growth of blackholes when they drive away gas,” says Hernquist. The outpouring of energy stunts both the black hole’s growth andstar formation in the galaxy’sbulge, in lockstep — leading tothe M-sigma relation. LeftoverTo view animations ofgas then settles down into a diskgalaxy mergers andof stars that resembles today’sactivity associated withspirals.supermassive blackThis furious feedback was aholes, visitSkyandTelescopecrucial component of the Millen.com/quasarnium Run, a simulation of thelargest virtual volume of spaceever attempted. A team led by Volker Springel (Max PlanckInstitute for Astrophysics, Germany) simulated the way inwhich matter clumped together inside a cube that is morethan 2 billion light-years on a side. The team started withthe tiny fluctuations in the distribution of matter encodedin the subtle temperature fluctuations of the cosmic microwave background. When gravity and dark energy acted onthose fluctuations for the simulated age of the universe,the result was a delicate network of galaxy clusters — thefamiliar cosmic web (see page 24).The Millennium Run incorporated feedback within andamong galaxies, such as supernova winds and powerfuljets of radio-emitting gas from supermassive black holes.The resulting shock waves dictated the fate of every galaxy. The simulation produced giant early galaxies withSloan-type quasars, but outflows from the black holes soonblasted away the surrounding gas. These gargantuan blackholes now reside within the “red and dead” giant ellipticalSky & Telescope July 2006 6
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black holes limited their own growth by unleashing torrents of energy that drove away the surrounding gas (S&T: April 2005, page 42). These waves of unrest also dictated the ebb and flow of starbirth in the host galax-ies. This feedback process forged a close link between massive black holes and their surrounding stars. This view of abrupt but dazzling mayhem in major galaxies represents a .
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