Dynamics and Evolution of Galactic Nuclei: 23 (Princeton Series in Astrophysics) - Softcover

Merritt, David

 
9780691158600: Dynamics and Evolution of Galactic Nuclei: 23 (Princeton Series in Astrophysics)
Softcover
ISBN 10: 0691158606 ISBN 13: 9780691158600
Verlag: Princeton University Press, 2013

Inhaltsangabe

Deep within galaxies like the Milky Way, astronomers have found a fascinating legacy of Einstein's general theory of relativity: supermassive black holes. Connected to the evolution of the galaxies that contain these black holes, galactic nuclei are the sites of uniquely energetic events, including quasars, stellar tidal disruptions, and the generation of gravitational waves. This textbook is the first comprehensive introduction to dynamical processes occurring in the vicinity of supermassive black holes in their galactic environment. Filling a critical gap, it is an authoritative resource for astrophysics and physics graduate students, and researchers focusing on galactic nuclei, the astrophysics of massive black holes, galactic dynamics, and gravitational wave detection. It is an ideal text for an advanced graduate-level course on galactic nuclei and as supplementary reading in graduate-level courses on high-energy astrophysics and galactic dynamics. David Merritt summarizes the theoretical work of the last three decades on the evolution of galactic nuclei, the formation of massive black holes, and the interaction between black holes and stars. He explores in depth such important topics as observations of galactic nuclei, dynamical models, weighing black holes, motion near supermassive black holes, evolution of nuclei due to gravitational encounters, loss cone theory, and binary supermassive black holes. Self-contained and up-to-date, the textbook includes a summary of the current literature and previously unpublished work by the author. For researchers working on active galactic nuclei, galaxy evolution, and the generation of gravitational waves, this book will be an essential resource.

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Über die Autorin bzw. den Autor

David Merritt is professor of physics at the Rochester Institute of Technology.

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"This direct, clear, and authoritative book shows Merritt's extensive experience with the techniques needed to understand the motions of stars in galaxies. It will be used as a reference by those who interpret the observations of stellar motions in galactic nuclei and will serve as a basis for further theoretical work."--Tim de Zeeuw, European Southern Observatory

"A leading expert on the dynamics of galactic nuclei, and of stars near massive black holes, Merritt has led many of the advances in our understanding of these systems. His very timely book fills a large gap in the literature of stellar dynamics and covers all the material that is required to embark on research in this field."--Tal Alexander, Weizmann Institute of Science

"Merritt is one of the most highly regarded astrophysical dynamicists in the field. Excellent, complete, and well-balanced, Dynamics and Evolution of Galactic Nuclei reflects his rigorous work."--Bradley Peterson, Ohio State University

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Dynamics and Evolution of Galactic Nuclei

By David Merritt

PRINCETON UNIVERSITY PRESS

Copyright © 2013 Princeton University Press
All rights reserved.
ISBN: 978-0-691-15860-0

Contents

Preface....................................................................ix
Chapter 1 INTRODUCTION AND HISTORICAL OVERVIEW.............................1
Chapter 2 OBSERVATIONS OF GALACTIC NUCLEI AND SUPERMASSIVE BLACK HOLES.....11
Chapter 3 COLLISIONLESS EQUILIBRIA.........................................57
Chapter 4 MOTION NEAR SUPERMASSIVE BLACK HOLES.............................117
Chapter 5 THEORY OF GRAVITATIONAL ENCOUNTERS...............................213
Chapter 6 LOSS-CONE DYNAMICS...............................................289
Chapter 7 COLLISIONAL EVOLUTION OF NUCLEI..................................361
Chapter 8 BINARY AND MULTIPLE SUPERMASSIVE BLACK HOLES.....................415
Suggestions for Further Reading............................................489
References.................................................................493
Index......................................................................535

CHAPTER 1

Introduction and Historical Overview


Of all the legacies of Einstein's general theory of relativity, none is more fascinatingthan black holes. While we now take their existence almost for granted, blackholes were viewed for much of the 20th century as mathematical curiosities withno counterparts in nature. Einstein himself had reservations about the existence ofblack holes. In 1939 he published a paper with the daunting title "On a stationarysystem with spherical symmetry consisting of many gravitating masses". Init, Einstein sought to prove that black holes—objects so dense that their gravityprevents even light from escaping—were impossible. Einstein's resistance to theidea is understandable. Like most physicists of his day, he found it hard to believethat nature could permit the formation of objects with such extreme properties.Ironically, in making his case, Einstein used his own general theory of relativity.That same theory was used, just a few months later, to argue the opposite case: apaper by J. Robert Oppenheimer and Hartland S. Snyder, entitled "On continuedgravitational contraction", showed how black holes might form.

The modern view—that black holes are the almost inevitable end result of theevolution of massive stars—arose from the work of Oppenheimer, SubrahmanyanChandrasekhar, Lev Landau, and others, in the first half of the 20th century. However,it was not until the discovery in 1963 of extremely luminous distant objectscalled quasars that the existence of black holes was taken seriously. What is more,black holes appeared to exist on a scale far larger than anyone had anticipated.

Quasi-stellar objects, or quasars, belong to a class of galaxies known as activegalactic nuclei, or AGNs. What makes these galaxies "active" is the emission oflarge amounts of energy from their nuclei. Moreover, the luminosities of AGNsfluctuate on very short timescales—within days or sometimes even minutes. Thetime variation sets an upper limit on the size of the emitting region. For this reasonwe know that the emitting regions of AGNs are only light-minutes or light-daysacross; far smaller than the galaxies in which they sit. At the time, astronomers werefaced with a daunting task: to explain how a luminosity hundreds of times that ofan entire galaxy could be emitted from a volume billions of times smaller. Of allproposed explanations, only one survived close scrutiny: the release of gravitationalenergy by matter falling toward a black hole. Even using an energy source as efficientas gravity, the black holes in AGNs would need to be enormous—millions oreven billions of times more massive than the Sun—in order to produce the luminositiesof quasars. To distinguish these black holes from the stellar-mass black holesleft behind by supernova explosions, the term "supermassive black hole" (SBH)was coined.

For nearly three decades after quasars were discovered, SBHs continued to beviewed as exotic phenomena and their existence was accepted only out of necessity.However, by the late 1980s, a crisis was brewing. Surveys with optical telescopeshad shown that the number of quasars per unit volume is not constant with time. Bystudying the redshift of the light emitted by the quasar on its journey to the Earth,astronomers found that the number density of quasars peaked when the universewas only about 2.5 billion years old and has been declining steadily ever since.

The reason for this evolution is still not completely understood. But whateverits explanation, the evolution presents astronomers with a challenge. Many of thequasars with large redshifts simply disappear at lower redshifts. Indeed, of thequasars that populated the skies almost 10 billion years ago, only one in 500 canbe identified today—but we know of no way to destroy the SBHs that powered thequasar activity. The unavoidable conclusion is that the local universe is filled with"dead" quasars, SBHs that have exhausted the fuel supply that made the quasarsshine so brightly 10 billion years ago.

Where are these dead quasars? A reasonable place to look is at the centers ofAGNs. But while the AGNs almost certainly contain SBHs, there are far too few ofthem—only a few percent of all galaxies are considered to be active—to accountfor the SBHs that once powered the quasars. By the early 1990s, astronomers werefaced with the prospect that an SBH might have to be located at the center of almostevery galaxy, which would make them as fundamental a component of galacticstructure as stars.

This idea—though natural enough—did not come easily, since most galaxiesshow no evidence for the emission associated with a central SBH. But the gravitationalfield of an SBH is strong enough to imprint a characteristic signature on themotion of surrounding matter at distances that are millions of times greater than theevent horizon. Stars, gas, and dust moving around a black hole—or any compactobject—have orbital velocities that follow the same laws discovered by JohannesKepler in the 17th century for the solar system. Moreover, the mass of the compactobject is easily computed once this Keplerian rotation has been mapped. Thesearguments have been applied to measure the mass of the SBH in the core of ourown galaxy, the Milky Way. But almost a decade earlier, the same was done for adistant galaxy, called NGC 4258.

NGC 4258 is a spiral galaxy, like the Milky Way, but containing an active nucleus.Sufficient radiation is produced in the nucleus to excite water molecules inthe gas clouds that orbit around it, resulting in strong, stimulated emission at radiowavelengths. These so-called water masers can be studied with very high spatialand velocity resolution using radio interferometric techniques. In 1994, it was reportedthat the maser clouds trace a very thin disk, which made their dynamicseasy to interpret. It was found that the motion of the clouds followed Kepler's lawto 1 part in 100, reaching a velocity of 1100 km s-1 at a distance of about one parsecfrom the center (figure 1.1). Only by assuming that the nucleus of NGC 4258hosts a central body with a mass 40 million times greater than the Sun could theseobservations be explained.

Perhaps even more remarkable is the case of the SBH at the center of the MilkyWay. The Galactic center has long been known to host a radio source, called SagittariusA* (Sgr A*), that is at rest, indicating that it must be very massive. But becauseof the high visible-wavelength extinction toward the Galactic center, almost70 years elapsed between the discovery of Sgr A* and the demonstration, usingground-based telescopes, that Sgr A* is in fact an SBH. Beginning around 1992,two groups, at the University of California at Los Angeles and the Max PlanckInstitute in Garching, have monitored the positions and velocities of over a thousandstars within a parsec of Sgr A*. The stellar motions have been reconstructedby combining the projected motion on the plane of the sky (the "proper motion")with the velocity along the line of sight; the latter was measured from the Dopplershifts of absorption lines in the stellar spectra. These data revealed the unmistakablefingerprint of an SBH: stars closer to Sgr A* move faster than stars farther away inthe exact ratio predicted by Kepler's law. Stars only a few light-days away from thesource move at fantastic speeds, in excess of 1000 km s-1. Such velocities can onlybe maintained if Sgr A* is roughly four million times more massive than our Sun.The current, best estimate of its mass is about 4.3 × 106 solar masses.

The Galactic center is 100 times closer than the next large galaxy, Andromeda,and 2000 times closer than the nearby association of galaxies, the Virgo Cluster.In no other galaxy do we have the opportunity to study the dynamics of individualstars orbiting a central SBH in such exquisite detail. To make matters worse, watermasers like the one that populates the nucleus of NGC 4258 are very rare, and evenmore rarely are they organized in simple dynamical structures that can be easilyinterpreted.

In each of these two cases, the data probe regions in which the stellar or gasmotions are completely dominated by the gravitational force from the SBH, just asthe motions of planets in the solar system are dominated by the force from the Sun.If we were to look further from the center of these galaxies, we would find that themotion of the stars and gas clouds is influenced more by all the other nearby starsthan by the central black hole.

In this regard, it is useful to define the "sphere of influence" of an SBH as theregion of space within which the gravitational force from the SBH dominates thatof the surrounding stars. The Galactic center stars, and the water masers in NGC4258, lie well inside the respective spheres of influence. Measuring the mass of anSBH from data that do not resolve the sphere of influence is a bit like judging theweight of a turkey that may, or may not, be lurking in a distant bush. The rustlingof the leaves may indicate a turkey; or it might be a bevy of quails; or maybe it'sjust the wind. All one can say for certain is that the bush isn't hiding an ostrich, oran elephant.

Largely with the help of the Hubble Space Telescope, we have now resolved thespheres of influence of the SBHs at the centers of a handful of nearby galaxies. Themost massive SBH detected to date, with a mass of about four billion solar masses,belongs to a giant elliptical galaxy M87 at the center of the Virgo Cluster.

Confident of the existence of SBHs, we can begin to ask more fundamental questionsabout them: How are SBHs related to their host galaxies? How did they form?What role do they play in galaxy evolution?

A partial answer to the first question emerged in 2000. A strong correlation turnsout to exist between SBHs and the properties of their host galaxies. The mass of anSBH can be predicted with remarkable accuracy by measuring a single number—thevelocity dispersion, σ, of the stars in the galaxy (figure 1.2). What is so surprisingabout this relation, aside from its precision, is that the stars whose velocitiesare measured are too far from the SBH to be influenced by its gravitational field. Inother words, SBHs appear to "know" about the motion of stars that lie well outsideof their sphere of mutual influence.

The origin of this relation is still being debated by theorists. But whatever its ultimatemeaning, the relation is an extremely valuable tool, because it linkssomething that is difficult to measure (the mass of an SBH) to something that iseasy to measure (the stellar velocity dispersion far from the SBH). This makes itpossible to determine the masses of SBHs in large samples of galaxies, much largerthan the sample for which the techniques described previously can be applied. Theresult was a new field of endeavor: black-hole demographics. One finds that about0.1% of a galaxy's stellar mass is associated with its SBH, and the average densityof SBHs in the local universe agrees remarkably well with the density inferredfrom observations of quasars. For the first time, the SBHs that powered the distantquasars were fully accounted for.

Current models of galaxy formation suggest that most large galaxies have experiencedat least one "major merger" during their lifetime: a close collision betweentwo galaxies that results in a coalescence (figure 1.3a). Computer simulations suggestthat when two spiral galaxies merge, the result is an elliptical galaxy: most ofthe gas is converted into stars, and the ordered rotation of stars in a disk is convertedinto the more random motions observed in elliptical galaxies. In a galactic merger,the SBHs at the centers of the two galaxies would sink rapidly to the center of themerged system through a process called dynamical friction. Once at the center, theywould form a bound pair—a binary SBH—separated by about a parsec. It has longbeen known that some active galaxies emit radio jets that twist symmetrically oneither side of the nucleus, suggesting that the SBH producing the jets is wobblinglike a precessing top. This is exactly what would happen in a binary SBH: the spinningSBH that produces the jets would precess as it orbits around the other SBH,just as the Earth's axis wobbles due to the gravitational pull of the Sun and theMoon.

Other active galaxies show periodic shifts in the amplitude or Doppler shift oftheir emission. The best-studied case, a quasar called OJ 287, has experienced severalmajor outbursts every 12 years since monitoring began in 1895. These flarescould be produced by a smaller SBH (108 solar masses) passing through the accretiondisk of a larger one (109 solar masses) once every 12 years. In the last fewyears, a number of other candidate binary SBHs have been found, some with apparentseparations as small as 10 parsecs. While the number of such cases is stillvery small, astronomers believe that most or all of the SBHs currently observed atthe centers of nearby galaxies must have been preceded by massive binaries.

As the two black holes in a binary system orbit each other, they emit energyin the form of gravitational waves, ripples in space-time that propagate outwardat the speed of light. Any accelerating mass produces this kind of radiation, butthe only systems that can produce gravitational waves of appreciable amplitudeare pairs of relativistically compact objects—black holes or neutron stars—in orbitabout each other. Gravitational waves carry away energy, and so a system emittinggravitational radiation must lose energy—in the case of a binary black hole, thismeans that the two black holes must spiral in toward each other. The infall wouldbe slow at first, but would accelerate until the final plunge when the two black holescoalesced into a single object. The coalescence of a binary SBH would be one of themost energetic events in the universe. However, virtually all of the energy would bereleased in the form of gravitational waves, which are extremely difficult to detect;there would be little if any of the electromagnetic radiation (light, heat, etc.) thatmake supernova explosions or quasars so spectacular.

No direct detection of gravitational radiation has ever been achieved, but theprospect of detecting gravitational waves from coalescing black holes is extremelyexciting to physicists: it would constitute robust proof of the existence of blackholes and it would permit the first real test of Einstein's relativity equations inthe so-called strong-field limit. Furthermore, by comparing the gravitational wavesof coalescing black holes with detailed numerical simulations, the masses, spins,orientations, and even distances of the two black holes could in principle be derived.

The prospect of observing the coalescence of a binary SBH is one of the primarymotivations behind building a gravitational-wave detector in space. Existing,Earth-based gravitational-wave detectors are not able to detect the long-wavelengthgravitational waves that would be generated by binary SBHs. A detailed designexists for a space-based detector, called the Laser Interferometer Space Antenna orLISA, which would consist of three spacecraft separated by five million kilometersflying in an equilateral triangle in the Earth's orbit. A passing gravitational wavewould stretch and squeeze the space between the spacecraft, causing very slightshifts in their separations. Although such shifts are tiny—some 10-12 m across—theycould be detected by laser interferometers.

The scientists who propose instruments like LISA must address one importantquestion: How frequently will the instrument detect a signal from coalescing blackholes? Space-based interferometers will have the sensitivity to detect mergers ofSBHs out to incredible distances, essentially to the edge of the observable universe.One way to estimate the event rate is to calculate how frequently galaxies mergewithin this enormous volume. On this basis, a telescope like LISA should detectat least one event every few years. However, the situation is more complicatedthan this, since it is only the final stages of black-hole coalescence that producean observable signal. In order to reach such small distances—less than 0.01 pc—theblack holes must first spiral together from their initial separation of severalparsecs. Gravitational radiation itself is too inefficient to achieve this; some othermechanism must first extract energy from the binary or else the decay will stall ata separation too great to generate a measurable signal for gravitational-wave telescopes.The prospect that some binaries might fail to close this gap has been calledthe "final-parsec problem." (Of course, it is a "problem" only from the standpointof the physicists who hope to detect gravitational waves.)

As the merging galaxies come together, the two SBHs fall rapidly to the centerof the merged system, dragged by the dynamical friction force acting on thegalaxies as a whole. Once the binary is in place, a new mechanism comes into playcalled the gravitational slingshot. Any star that passes near to the massive binaryis accelerated to high velocities and ejected, taking energy away from the binaryand causing its orbit to decay slightly. As a result, the separation between the SBHsgradually shrinks, although perhaps not enough to place the massive binary into thegravitational-wave regime.

There is good observational support for this model. Ejection of stars from agalactic nucleus by a massive binary would drastically lower the density of starsthere, on a spatial scale roughly equal to the gravitational influence radius of thelarger SBH (figure 1.4). Just such a feature—a low-density core, or "mass deficit"—isalways found in bright elliptical galaxies, which statistically should have experiencedthe most mergers.

These cores are formed at an early stage in the evolution of the binary, and theirpresence does not necessarily imply that the two SBHs coalesced. If coalescencewere delayed, long enough for a third SBH to be brought in by a subsequent galaxymerger, the three massive objects could undergo a gravitational slingshot. Theresulting violent interaction could eject one or more of the black holes from thenucleus, and possibly from the entire galaxy. In this way, rogue SBHs might becreated that drift forever between the galaxies.

Another prediction concerns the spins of black holes. If two SBHs coalesce, theirorbital motion during the final plunge is converted into rotation of the resultingobject. This means that SBHs at the centers of galaxies should be rotating rapidly.Furthermore, the directions of their spin axes should be essentially random, sincethe mergers responsible for imparting the spin take place from random directions.This prediction is consistent with observations of the orientations of radio jets inactive galaxies, which are thought to point in the same direction as the spin axis ofthe black hole: the jet orientations are random with respect to the orientations oftheir host galaxies.


(Continues...)
Excerpted from Dynamics and Evolution of Galactic Nuclei by David Merritt. Copyright © 2013 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Verlag
Princeton University Press
Erscheinungsdatum
2013
Sprache
Englisch
ISBN 10 
0691158606
ISBN 13 
9780691158600
Einband
Tapa blanda
Anzahl der Seiten
568
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