Galaxy evolution Exploring the galaxy-black hole connection Every large galaxy seems to have a supermassive black hole at its heart. Yet no one knows exactly how they feed off each other. by Steve Nadis Beautiful barred spiral galaxy NGC 1672 harbors a supermassive black hole at its center — as do all large galaxies astronomers have studied. The light from billions of stars masks the black hole. NASA/ESA/Hubble Heritage Team (STScI/AURA) nderstanding family dynamics black holes — those possessing the mass precise, it was between black holes and By 2000, the fit looked even better, can be a daunting and sometimes of millions, if not billions, of suns — are elliptical (football-shaped) galaxies and as two teams of astronomers found a hopeless task. The influences are not rare accessories that adorn only the between black holes and the bulges (cen- correlation between black hole mass often subtle, the agendas hidden, most exclusive galaxies. Instead, every tral concentrations) in spiral galaxies. and “velocity dispersion” — the random and the motives tangled. And large galaxy astronomers have studied Observations have fixed this mass velocities of stars orbiting within the U history shades events in almost imper- in detail has an enormous black hole ratio at somewhere between 0.1 and 0.2 bulge. What makes this correlation par- ceptible ways. What holds true for human lurking near its center. This one-to-one percent. Caltech astronomer Dominik ticularly striking is that the majority of families also may apply to families of the correspondence between large galaxies Riechers narrows it down further, to stars in the bulge lie far enough from the cosmic sort, such as the “families” com- and black holes suggests a fundamental about 1⁄700 (0.14 percent). “Since we see black hole to be totally immune from prising central black holes and their link between these two cosmic entities. the same ratio over a range of galaxies of its gravitational influence, explains Kor- A high-speed jet of subatomic parent galaxies. different masses and at different stages of mendy, a member of the team headed by particles beams from the giant elliptical galaxy M87 in Virgo. Throughout the 1990s, astronomers Fit to be tied evolution, we assume there must be some his Texas colleague Karl Gebhardt. The source of this bluish jet is a increasingly became aware of the inti- In 1993, University of Texas astronomer mechanism that regulates the growth of “If there’s a tight correlation between supermassive black hole in the mate, though enigmatic, connections that John Kormendy uncovered a correlation the black holes and galaxies, as well as the black hole mass and some property of galaxy’s core. exist between black holes and galaxies. between the mass of a central black hole some way of communicating between the galaxy that doesn’t even know a black NASA/ESA/Hubble Heritage Team (STScI/AURA) Team NASA/ESA/Hubble Heritage One vital realization is that supermassive and that of its host galaxy. To be more them,” says Riechers. hole is there,” says Kormendy, “that really © 2010 Kalmbach Publishing Co. This material may not be reproduced in any form 28 Astronomywithout • May permission 2010 from the publisher. www.Astronomy.com www.Astronomy.com 29 and help solve some of the existing 12.7 billion years ago Current universe conundrums. So far, the empirical data is painting a more nuanced picture than what astronomers might have drawn a decade ago. In particular, the research has raised key questions about the so- called lock-step scenarios in which black holes and galaxies waltz through cosmic history hand in hand. Peering into the abyss In a February 2009 paper, Yale University astronomer Kevin Schawinski and col- leagues cast doubt on what arguably had been the leading mechanism advanced to account for black hole-galaxy coevo- lution. The standard explanation goes something like this: As a black hole gravi- Computer simulations show the intimate relation between black holes, quasars, and galaxies. A simulation of the universe a billion years after the Big Bang (left) reveals black holes with masses greater than a billion Suns already had formed and were the driving forces behind the first quasars. A snapshot of today’s tationally pulls in surrounding material, universe from the same simulation (right) shows that those quasars evolved into the most massive galaxies at the centers of the biggest clusters. V. Springel (MPIA) et al. the gas circles the abyss faster and faster as it approaches the black hole’s event horizon — the point of no return. The speaks to the notion of ‘coevolution.’” Douglas Richstone. “The correlations puter simulations. They’re still far from material forms an accretion disk around Coevolution implies that the evolution that we’ve observed in the nearby uni- being able to make firm predictions the black hole, and friction within the of black holes and galaxies is some sort verse probably exist at earlier epochs than about how black holes and galaxies inter- disk makes it incredibly hot. of tandem process in which the two par- we’ve documented so far, but it’s still a act and evolve, says Abel, “because you The disk becomes a powerful source Twin jets erupt from the active ties “communicate” through feedback. deep mystery as to how these correlations have to understand all the things that of X-rays and other radiation as well as a nucleus of the Seyfert galaxy Since 2000, Kormendy adds, “all sorts of developed and when they developed.” happen in the middle of galaxies over trigger for high-speed jets that spew out 3C 219. The bright dot at the people have tried to come up with theo- The problem is not that scientists the course of billions of years in order matter. At the peak of its feeding frenzy, galaxy’s center is emission from ries for how black holes and galaxies talk don’t have a theory, Richstone adds. “If to have a truly predictive model.” the black hole gives birth to a quasar, one material swirling around the to each other, and how that talking cre- anything, we have too many theories. If That’s a tall order, and it’s no surprise of the brightest types of objects in the central black hole. NRAO/AUI ates this correlation.” you go to five different doctors and get that theory has lagged behind observa- universe and the most luminous kind of The start of the new millennium five different diagnoses, you might get tions. Meanwhile, observers are trying to active galactic nucleus (AGN). When the brought a dawning sense that scientists a little suspicious.” whittle away at existing uncertainties. For black hole accretes matter at a more lei- out strong bursts of high-energy X-rays. was made, but you know the layer it’s in can’t understand black holes without Most of what astronomers know today instance, they are busy determining the surely pace, this central engine becomes Schawinski’s team wanted to find out and the age of the layer. We use the stars understanding galaxies. A picture has come from observations rather than mass correlations with ever-increasing a more pedestrian AGN. what the galaxies were up to when the themselves as the ‘layer’ to figure out emerged of black holes and their galactic theory, says Stanford astrophysicist Tom accuracy. That information, in turn, The theory holds that all the energy AGNs were most active. when lots of star formation occurs.” hosts growing together, in “lock step” as Abel, a theorist who specializes in com- could lead to better theoretical models emitted by the AGN would clear out The approach shows that AGNs reach it were. All that remained was to explain stray material from the vicinity of the How green is my valley their most active state in green galaxies, exactly how this happened. Forging the galaxy connection black hole. This would curtail the black Astronomers long ago recognized that about 100 million years after star forma- hole’s own growth while simultaneously young star-forming galaxies are blue. tion crested in the earlier, blue period. By Mass from star motions Not so fast Mass from hot-gas motions heating any nearby gas so it could no lon- This occurs because short-lived, massive this reckoning, AGNs cannot shut down A decade has passed, and one might Mass from cold-gas motions ger condense into stars. In this way, the stars dominate them, and those behe- star formation because star formation think that researchers would have worked 1 billion AGN would regulate the growth cycles moths burn hot and emit most of their winds down long before the AGNs turn out the details of black hole-galaxy inter- of both the black hole and the galaxy — visible light in the blue part of the spec- on. If Schawinski and his collaborators actions. Unfortunately, that is not the they would keep on growing until the trum. These stars die off within a few are right, the conventional scenario is off case. Although scientists have made 100 million AGN spewed out enough energy to shut million years and are gradually replaced in some critical details. “This whole inter- progress on the observational front, an them both down. by smaller and cooler ones, which give play — this coevolution between black over arching theory that weaves together Although this theory still has many off more red light. holes and galaxies — may be more com- the various threads remains elusive. 10 million adherents, Schawinski and company sug- But the rapidly growing black holes plicated than we once thought,” he says. “Everyone is fumbling around, trying gest that the above chronology doesn’t spotted by the Yale-led group inhabit Another 2009 paper seems to corrob- to see what the next big insight will be,” Mass of black hole (solar masses) quite add up.