Black Holes & Galaxies
Image of the center of the Milky Way Galaxy including the X-Ray source associated with its supermassive black hole.
Black holes are some of the most mysterious and intriguing objects in the universe. Gravitational forces near them are so strong that nothing — not even light — can escape their fatal attraction. Small and intermediate-sized black holes are formed in the cataclysmic explosions that mark the death of large-to-massive stars. Supermassive black holes (millions to billions times more massive than our Sun) exist in the center of galaxies, including the Milky Way. How they form and their role in the creation and maintenance of galactic structures is under study.
Phil Armitage conducts theoretical studies of the interaction of black holes with their surroundings. He is interested in theories that explain the causes of star formation within massive accretion disks orbiting black holes and the coalescence of planetary systems from disks of gas and dust orbiting new stars. He is currently evaluating a "gravitational collapse" theory, which posits that gravitational interactions in an accretion disk cause clumps of gas to form. These clumps then attract more matter to them, eventually forming stars or planets.
Many scientists believe this mechanism is most likely to occur in the outer regions of a supermassive black hole than in a planetary accretion disk. If so, the gravitational collapse theory may explain why there are often a multitude of stars near black holes. For instance, two egg-shaped necklaces of magnificent stars were created about six million years ago around the black hole at the center of the Milky Way Galaxy. A similar cluster of massive stars orbits Andromeda’s central black hole. Armitage has designed a simulation based on the gravitational collapse theory that shows brilliant stars like these being created when gaseous disks in the process of being pulled into orbit around a black hole fragment into stars. The black-hole-mediated process is altogether different from star creation in cold gas clouds elsewhere in a galaxy.
Armitage has also explored another interesting phenomenon occurring around the black holes in some galactic centers. In several galaxies, disks of water masers orbit black holes like a planetary system, within a distance of half a light year. The hot (600 K) water molecules mase (emit coherent radio wavelength photons) when they return to lower energy states after being excited by collisions inside gas clouds spiraling toward the black hole. The masers eventually get pulled into the supermassive black hole, but long before this happens, their orbits become circular.
Blazar Galaxy
Mitch Begelman probes the origins of massive black holes in the center of galaxies. He wants to understand how massive black holes regulate the structure of galaxies and the evolution of galaxy clusters. He undertakes many different research projects that probe aspects of these important processes, including studies of jets created by black holes, the release energy from black holes, and black-hole mergers that occur when galaxies collide. He uses sophisticated computer simulations to model such complex processes. Recently, he and his collaborators from Cambridge University and Princeton figured out why under certain conditions inside blazer galaxies ultrahigh-energy ?-rays travel away from massive black holes inside high-speed jets. The ?-rays appear to be created during explosions inside the jets. The explosions release tension produced by the presence of very high (and energetic) magnetic fields in the jets. These explosions create a "jet within a jet."
Begelman collaborates with Armitage on a sophisticated simulation to test Begelman’s ground-breaking hypothesis explaining the origin of the supermassive black holes found at galactic centers. Begelman’s idea posits that the seeds for colossal black holes were sown during the initial formation of galaxies about a billion years after the Big Bang. The black holes formed at what had been the centers of huge, dense reservoirs of dark matter coupled to ordinary matter (mostly hydrogen gas). This coupling broke down when the dense clouds of hydrogen gas cooled sufficiently to begin falling into the center under their own gravity. The in-fall process eventually led to the creation of a dense, self-gravitating core supported by intense radiation pressure.
The cores resembled gigantic stars, except that they never quite reached equilibrium. The in-falling gas continued to compress these cores until they ran out of nuclear fuel and lost the ability, after a couple of million years, to resist the intense gravitational pressures. At this point, the cores collapsed into black holes with masses ranging from tens of thousands to several million Suns
The black holes then began sucking in the leftover mass of their envelopes at the incredible rate of about 10 solar masses per year. Hot matter escaping the black holes’ gravitational pull puffed up these envelopes into gigantic quasi stars resembling red giants, expanding them at least a hundredfold in less than a million years. Eventually these yellow giant stars evaporated, leaving behind black holes with masses between 100,000 and 10,000,000 Suns. Begelman believes these primordial black holes were the seeds that gave rise to the supermassive black holes (ranging in size from 100,000 to 10 billion Suns) seen today in every galaxy in the Universe with a bulging center.
Simulated interaction of a gas disc with two black holes in close orbit around one another.
In addition to investigating the origin of primordial black holes, Begelman works with Armitage on modeling the merger of black holes when galaxies collide. In 1980, Begelman (then at the University of California at Berkeley) and colleagues from Caltech and the United Kingdom’s Institute of Astronomy partially explained how these mergers occur: As they move toward the center of the new galaxy, the two black holes “bump” into stars, knocking them out of the way. The black holes lose energy and move closer together until they are about 0.1–0.01 light years apart. At this point, there aren’t any stars left to lob out of the galactic center.
In 2008, Begelman and Armitage used high-speed supercomputers to show that if the two black holes have masses of less than 10 million Suns, additional interactions with gas clouds would then bring them close enough to merge. However, this mechanism failed to promote mergers between larger black holes. The simulation required so much gas to bring the bigger black holes together that stars formed in the gas cloud instead. Thus, a third process must be involved in the merger of more massive black holes, possibly their interaction with the new stars formed during the fragmentation of a large gas disk.
Gravitational waves could provide evidence of the astrophysical processes that allow supermassive black hole binaries to coalesce. In the future, the Laser Interferometer Space Antenna (LISA), a joint mission being planned by NASA and the European Space Agency, should be able to gather this evidence.
The Laser Interferometer Space Antenna (LISA)
Prior investigations at JILA by Peter Bender and Jim Faller have made important contributions to the design of LISA. Bender is now working with students and scientists at other institutions to improve the accuracy achievable by LISA and ensure its reliability. LISA's main objectives are to observe gravitational waves from massive black hole binaries formed in galaxy mergers and study the formation and growth of massive black holes and the galaxies associated with them. To detect gravitational waves, precision laser measurements will be made between carefully protected test masses in three spacecraft that form an equilateral triangle in orbit around the Sun. Each side of the triangle will be approximately 5 million kilometers long.
One challenge during LISA's mission will be to make sure that phase noise in the lasers does not limit the accuracy of the gravitational wave measurements. With precise phase measurements on the laser beams sent between the spacecraft, the effect of laser phase noise can be cancelled out in determining changes in the ratios of the lengths of the sides of the triangle, as pointed out originally by Faller. However, the reduction of laser phase noise by locking the laser tightly to a stable reference cavity helps to provide robustness for the overall phase measurement system. To achieve lower phase noise, researchers are currently testing a new cavity design with low sensitivity to vibration.
Bender is working with colleagues around the world to prepare for the LISA mission. A LISA Pathfinder mission, funded mainly by the European Space Agency, is scheduled for launch in 2011. The Pathfinder’s primary goal is to verify the (expected) extremely low level of residual test-mass accelerations with laser interferometric measurements between the test masses in two gravitational reference sensors on the same spacecraft.
Images of galaxies (taken by the Hubble Space Telescope) emitting X-ray flashes. The galaxies appear very small - about the size of a penny viewed from 5 miles away - because of their distance. The bottom panels show a close up of the galaxies with circles representing the Chandra X-ray positions of the "afterglow" light from the flashes.Rosalba Perna studies gamma-ray bursts and their afterglows at longer wavelengths. Gamma-ray bursts, which occur billions of light years from Earth, are among the most intriguing astronomical phenomena of modern times. They emit a hundred billion times more energy in a few seconds than our Sun produces in a year. These luminous events signal the formation of a black hole during a supernova. Perna uses gamma ray bursts and their afterglows to probe star and galaxy formation in the early (high red-shift) Universe. Because they are more luminous than quasars, gamma-ray bursts can penetrate the dense and dusty stellar nurseries that produced the Universe's earliest generations of stars.
Perna is a collaborator on teams using the Swift satellite. The teams have (1) discovered the oldest, most distant gamma-ray burst to date, (2) identified the optical afterglow of a short gamma-ray burst of the type that is due to mergers between two neutron stars or a neutron star and a black hole, and (3) detected the presence of X-ray flares in the early afterglows of gamma-ray bursts. She recently collaborated with Phil Armitage on a model proposing that fragmenting accretion disks give rise to these flares.
Perna also studies anomalous X-ray pulsars and soft gamma-ray repeaters, a subclass of neutron stars whose X-ray emission is believed to be powered by the dissipation of a very strong magnetic field.
Andrew Hamilton works on modeling the growth of supermassive black holes inside galaxies. He and his colleagues have created a simulation that can model physics over a large dynamic range, starting with the formation of a large gas-cloudlike structure and zooming into the outer edge of a black-hole accretion disk in the center of the galaxy. They began with a study of gas behavior in the center of the galaxy early in the galaxy’s history. They found a relatively stable, but turbulent, self-gravitating and rapidly rotating accretion disk. The disk appeared to continuously feed the central black hole without being interrupted by fuel-depleting bursts of star formation. The researchers are now investigating black-hole growth at different times during the history of the galaxy, including one in which this galaxy merges with another galaxy. Eventually, they plan to compare black-hole growth in the simulated galaxy with observations of active galactic nuclei.
Andrew Hamilton’s conception of the inside of a black hole is a visualization of mathematical equations written by Albert Einstein to describe gravity. It's the first attempt to use real science to show the inside of a black hole.
Hamilton also explores the implications of Einstein's theory of relativity for black-hole structure and behavior. He proposed a model for the flow of matter into a black hole, in which space flows like a river through a flat background. Objects like light rays that move through the river of space abide by the rules of special relativity. When the river of space falls into a stationary black hole, it flows toward the singularity faster than light, bringing along everything inside it. In a rotating black hole, the river of space also twists. As objects move in the river, they are buffeted by tidal changes in the speed and rotation of the river of space. A photon in the river would experience these changes as an evolving curvature of space-time.
Hamilton also suggested that black holes are not mostly empty, as previously thought. According to his analysis, the space between a black hole's outer edge, or event horizon, and the edge of its inner core, or inner horizon, is mostly empty. However, the core is filled with hot relativistic plasma generated by the violently unstable inner horizon. The singularity lies nestled inside the dense, hot plasma. His model confirms a 1990 prediction by Poisson and Israel of an unstable inner horizon due to relativistic waves of baryons streaming toward the inner horizon and waves of dark matter counter-streaming back toward the outer horizon. It also has implications for how a person falling into a black hole would be killed: Tidal forces on a river of space flowing into a stellar-sized black hole would stretch out a human being until she was as thin as spaghetti, tearing her apart in the process. In contrast, a person falling into a supermassive black hole, where tidal forces are weak, would be fried by the plasma.
Hamilton used his deeper understanding of black-hole structure and behavior to create a Black Hole Flight Simulator, which he used in his role as science advisor to multimedia black hole productions by the Denver Museum of Nature and Science, NOVA, and the National Geographic Channel.
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