Monster black holes.
Armed with ultrasensitive light detectors and state-of-the-art telescopes, astronomers during the past decade have peered deeper than ever before into the center of galaxies. And there they've found more than just stars. Some galaxy cores rank among the most brilliant and energetic entities in the universe. Spewing out jets of radiation that extend for thousands of light-years, the cores of active galaxies can accelerate particles to energies dwarfing those of the biggest accelerator on Earth.
Powerful cores require powerful energy sources. Many astronomers now believe that compact, supermassive black holes may fuel the fireworks fireworks: see pyrotechnics.
Explosives or combustibles used for display. Of ancient Chinese origin, fireworks evidently developed out of military rockets and explosive missiles and accompanied the spread of military explosives westward to at the center of some galaxies. Supermassive black holes are dense, collapsed objects--millions to billions of times the mass of the sun - with a gravitational grav·i·ta·tion
a. The natural phenomenon of attraction between physical objects with mass or energy.
b. The act or process of moving under the influence of this attraction.
2. tug so strong that not even light can escape their clutches. "The only way we know how to put such a big mass in a very small region is to make it a black hole," says Alan Dressler of the Observatories of the Carnegie Institution in Pasadena, Calif.
But while black holes have captured the imagination of scientists and the public alike, hunting them remains a challenge. By definition, a black hole can't be seen. This means its presence must be detected through indirect evidence. So, like an invisible houseguest who makes his presence known by the amount of food he eats, a black hole reveals its existence by the way it influences its surroundings.
Yet the mounting flurry of supporting evidence for galactic black holes can leave the average astronomy buff bewildered. Reports earlier this year about black hole candidates - many based on observations with the Hubble Space Telescope Hubble Space Telescope (HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe. - beg some fundamental questions: What are the criteria for finding a supermassive black hole, and what constitues the nearest thing to absolute proof?
Just the place for a snark snark
elusive imaginary animal. [Br. Lit.: The Hunting of the Snark]
See : Quarry
snark - [Lewis Carroll, via the Michigan Terminal System] 1. A system failure. , I have said it twice, That alone should encourage the crew. Just the place for a snark, I have said it thrice thrice
1. Three times.
2. In a threefold quantity or degree.
3. Archaic Extremely; greatly. , And what I tell you three times is true. -"The Hunting of the Snark"
Just as the hunters of Lewis Carroll's mythical beast had their own standards for deciding when they had found the creature, astronomers have developed some basic rules for determining when they have snared the black hole they seek. To house a giant black hole, a galactic core must meet two requirements. It must exhibit a telltale increase in the intensity of starlight from its edge to its center. And the stars surrounding the core must orbit so rapidly that the tug from visible matter alone can't account for their speed.
While searching for bright nuclei may appear easier than measuring star velocities, the tests must go hand in hand to prove the existence of a galactic black hole, says John Kormendy of the University of Hawaii (body, education) University of Hawaii - A University spread over 10 campuses on 4 islands throughout the state.
See also Aloha, Aloha Net. in Honolulu. Adds Dressler: "Most astronomers I know are not saying any black hole is absolutely proven until the kinematic kin·e·mat·ics
n. (used with a sing. verb)
The branch of mechanics that studies the motion of a body or a system of bodies without consideration given to its mass or the forces acting on it. data - the motions of stars - complement studies of light intensity."
Astronomers often trace the hunt for galactic black holes to 1978, when the late astronomer Peter Young and his colleague Wallace L. W. Sargent Wallace Leslie William Sargent (born 1935) is an American astronomer. He is often known as Wal Sargent. Although now a U.S. citizen, he was born in Elsham, England. He received his Ph.D. of the California Institute of Technology California Institute of Technology, at Pasadena, Calif.; originally for men, became coeducational in 1970; founded 1891 as Throop Polytechnic Institute; called Throop College of Technology, 1913–20. in Pasadena published two seminal papers on the giant elliptical galaxy M87. The researchers suggested that an unexpected increase in the intensity of light toward the galaxy's center might signify a collection of densely packed stars orbiting a black hole. They also asserted that as higher-resolution telescopes became available, these instruments would find that the concentration of starlight closer and closer to the galaxy's center would continue to rise steeply, according to a formula that describes the density of stars surrounding a black hole.
The research articles, recalls Kormendy, drew criticism from many astronomers. But the studies also sparked fresh interest in the black hole scenario.
In their analysis, Young and Sargent had assumed that near the core of M87, located some 52 million light-years from Earth, stars move with about the same average velocity in all directions. Astronomers later found that this so-called isotropic Refers to properties that do not differ no matter which direction is measured. For example, an isotropic antenna radiates almost the same power in all directions. In practice, antennas cannot be 100% isotropic. distribution of stars is not correct for most giant elliptical galaxies and probably does not describe the motions of stars in M87. Researchers pointed out that if stars near the center of M87 had a less isotropic distribution, moving nearly straight in and straight back out from the core, then their light profile and velocities would mimic the star movement and light intensity expected if a black hole were present.
Last January, researchers announced that the Hubble Space Telescope had examined the core of M87 at a resolution greater than that of ground-based telescopes. The Hubble study confirmed that the intensity of starlight increased toward the center, as Sargent and Young had predicted (SN: 1/25/92,p.52). Sandra M. Faber Sandra Moore Faber (1944 - ) is a University Professor of Astronomy and Astrophysics at the University of California, Santa Cruz and works at the Lick Observatory. In 1972 she received her Ph.D. in Astronomy from Harvard University, prior to that she obtained a B.A. of the University of California, Santa Cruz The University of California, Santa Cruz, also known as UC Santa Cruz or UCSC, is a public, collegiate university, one of the ten campuses of the University of California. , a Hubble team member, says her group's study makes the case for a billion-solar-mass black hole in M87 more compelling, but not airtight. Alternative explanations for the light intensity don't require a black hole, but she notes that these models do require an asymmetric distribution of stars so unusual that she regards them as "hokey hok·ey
adj. hok·i·er, hok·i·est Slang
1. Mawkishly sentimental; corny.
2. Noticeably contrived; artificial.
Dressler agrees that such models often seem contrived. And given the fiery glow of starlight from M87, as well as a bright jet of material spewing out from its center, he says a black hole probably does lurk at M87's core. But Dressler adds that without further evidence, models that don't invoke a black hole can't be dismissed just because they seem unlikely. "Extraordinary claims require extraordinary proof," he says.
To find a supermassive black hole, notes Kormendy, astronomers must "weigh" the center of a galaxy. Measuring light intensity doesn't accomplish the task directly. Instead, Kormendy, Dressler, and their colleagues rely on kinematic evidence.
Using spectroscopy, they measure the velocity of stars whipping around a galactic nucleus. Applying Newton's law of gravitation Newton's law of gravitation: see gravitation.
Newton's law of gravitation
Statement that any particle of matter in the universe attracts any other with a force (F) that is proportional to the product of their masses (m1 , the scientists then estimate the amount of mass that must be present to account for such rapid motion. By recording the intensity of starlight at the center, the researchers can calculate how much of the required mass is associated with visible stars. If the mass contained in visible stars isn't enough, then there must be additional, invisible material concentrated in a tiny region: precisely the criteria for a galactic black hole.
Following this relatively straightforward recipe, Kormendy, Dressler, Faber, and their colleagues had hoped to use Hubble to pin down the galaxies most likely to harbor black holes. But Hubble's blurry optics have -- at least temporarily -- dashed that hope. In its present condition, the telescope can record the intensity of starlight, but it can't perform the high-resolution spectroscopy required to measure the motion of stars at the core of galaxies.
Ground-based studies, however, have made the black hole scenario a compelling model for several galaxies, Kormendy contends. Ironically, all of them have relatively quiescent cores, exhibiting none of the fireworks associated with an active quasar quasar (kwā`sär), one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and currently believed to be the most distant and most luminous objects in the universe; the name is shortened from or other powerhouse that might require a black hole.
Two of the galaxies--our nearest spiral neighbor, Andromeda, and its satellite galaxy M32 -- lie close enough to the Milky Way for ground-based spectroscopic spec·tro·scope
An instrument for producing and observing spectra.
spectro·scop studies. Telescopes on Earth can also monitor the motion of stars at the core of certain more distant neighbors, such as NGC NGC New General Catalogue (of Nebulae and Star Clusters; astronomy)
NGC National Geographic Channel (TV)
NGC National Guideline Clearinghouse 3115, because the black holes believed to reside there rank among the biggest of the current candidates.
In the mid-1980s, while studying the far more luminous core of another galaxy, NGC 1068, Dressler stumbled onto evidence that nondescript Andromeda might harbor a black hole. Although NGC 1068 seemed like a prime candidate for containing a compact object, Dressler's observations turned the tables on his prediction.
"I took the spectrum of [Andromeda] merely as a calibration, to make sure I knew what the center of a galaxy looks like where there wasn't a black hole," recalls Dressler. "I didn't see anything special in NGC 1068, but I saw this amazingly rapid rotation of stars [at the core of Andromeda]." In a separate study, Kormendy also found evidence that a supermassive black hole could best explain the motion of stars at Andromeda's core. Dressler and Kormendy say they consider the core of Andromeda the most likely candidate for a black hole.
According to Kormendy, the galaxy NGC 3115 ranks second in the black hole sweepstakes. Located some 30 million light-years from Earth, this neighboring galaxy has a dense cluster of stars -- within a few hundred light-years of the center -- that rotates rapidly about the core. Observations with the Canada-France-Hawaii telescope atop Mauna Kea show that the motion is so fast that the mass of the visible stars can't explain it -- not by a long shot.
Although the apparent runner-up as far as kinematic evidence goes, NGC 3115 appears to be the heavyweight champ among the leading black hole contenders, with a mass more than a billion times that of the sun. Kormendy and Douglas O. Richstone of the University of Michigan (body, education) University of Michigan - A large cosmopolitan university in the Midwest USA. Over 50000 students are enrolled at the University of Michigan's three campuses. The students come from 50 states and over 100 foreign countries. at Ann Arbor reported their work in the July 10 ASTROPHYSICAL JOURNAL.
Andromeda's tiny satellite M32 garners third place among galaxies most likely to contain a black hole, Kormendy suggests. Again, the discovery was made by serendipity serendipity
happy finding of an unexpected object or solution while searching for something else. . John L. Tonry of the Massachusetts Institute of Technology Massachusetts Institute of Technology, at Cambridge; coeducational; chartered 1861, opened 1865 in Boston, moved 1916. It has long been recognized as an outstanding technological institute and its Sloan School of Management has notable programs in business, examined the general character of this small spherical galaxy, believed to be the remnant of a larger galaxy that was stripped of material by the gravitational tug of Andromeda.
His 1987 study revealed stars moving so rapidly that the mass of visible stars estimated to lie at the galaxy's core couldn't explain their motion. In addition to the spectroscopic evidence, recent Hubble images have revealed that the core of M32 emits an unusually high intensity of starlight, indicating that the tiny galaxy may harbor a black hole about 3 million times the mass of the sun (SN: 4/18/92,p.245).
Dressler notes that, aside from Andromeda's large central bulge, the galaxy appears "kind of common, kind of dull." Thus, "if Andromeda, the closest spiral galaxy to us... has a black hole, then it does imply that a black hole is probably in every galaxy with a big spheroidal spheroidal /sphe·roi·dal/ (sfer-oi´d'l) resembling a sphere.
resembling a sphere. bulge," including the Milky Way (see sidebar, p.296).
Dressler suggests that some of these bulging galaxies -- those with the most massive cores -- once shone with the blinding light of quasars Proper naming of quasars are by Catalogue Entry, Qxxxx±yy using B1950 coordinates, or QSO Jxxxx±yyyy using J2000 coordinates.
This page lists quasars.
Of or relating to an extreme condition in which matter is forced into nonclassical states, as when electrons are forced into protons, leaving only neutrons, or the matter is compressed beyond this point into a singularity. bodies that gave them their energy. He estimates that up to half the galaxies in the universe may harbor supermassive black holes.
Ironically, he adds, researchers shy away from Verb 1. shy away from - avoid having to deal with some unpleasant task; "I shy away from this task"
avoid - stay clear from; keep away from; keep out of the way of someone or something; "Her former friends now avoid her" galaxies with highly energetic cores, some of the most likely places to look for supermassive black holes. Instead, they prefer to hunt their quarry among some of the least active galaxies -- provided the galaxies have a central bulge. Galaxies with quiet cores, notes Dressler, don't spew out jets of material that can outshine out·shine
v. out·shone , out·shin·ing, out·shines
a. To shine brighter than.
b. To be more beautiful, splendid, or flamboyant than.
2. starlight and confound measurements of stellar motion.
If Andromeda, its satellite, and NGC 3115 rank among the likeliest candidates for housing black holes, what would it take for astronomers to obtain compelling proof?
Because the radius of a supermassive black hole is no bigger than the solar system, the ability to watch stars begin to fall toward such an object -- even in a nearby galaxy -- would require a telescope a million times sharper than now exists, Kormendy notes. Lacking such an instrument, astronomers had looked to Hubble to gather more convincing proof. Indeed, Dressler notes, Andromeda and its satellite were prime targets for spectroscopic studies with Hubble.
"We thought we'd actually go in and nail them [as having central black holes], getting that extra factor of 10 in resolution that would eliminate any other contrived explanation," Dressler says.
Such studies will probably have to wait until astronauts correct Hubble's optics late next year. But over the next several years, Kormendy says, ground-based telescopes atop such choice viewing sites as Hawaii's Mauna Kea may nearly match a repaired Hubble.
Until then, contends Alexei V. Filippenko of the University of California, Berkeley The University of California, Berkeley is a public research university located in Berkeley, California, United States. Commonly referred to as UC Berkeley, Berkeley and Cal , "extragalactic ex·tra·ga·lac·tic
Located or originating beyond the Milky Way.
Adj. 1. extragalactic - outside or beyond a galaxy; "extragalactic nebula" black holes will remain like Darth Vader, cloaked in a shroud of circumstantial evidence circumstantial evidence
In law, evidence that is drawn not from direct observation of a fact at issue but from events or circumstances that surround it. If a witness arrives at a crime scene seconds after hearing a gunshot to find someone standing over a corpse and holding a ."