Wednesday, December 07, 2011

Pair of black holes 'weigh in' at 10 billion Suns; most massive yet

An artist's conception of stars moving in the central regions of a gaint elliptical galaxy that harbors a supermassive black hole. (Credit: Gemini Observatory/AURA artwork by Lynette Cook). JPEG (1393 KB)

Karl Gebhardt is the Herman and Joan Suit Professor of Astrophysics at The University of Texas at Austin. (McDonald Observatory/UT-Austin)

AUSTIN — A team of astronomers including Karl Gebhardt and graduate student Jeremy Murphy of The University of Texas at Austin have discovered the most massive black holes to date — two monsters weighing as much as 10 billion suns and threatening to consume anything, even light, within a region five times the size of our solar system.

The research is published in the December 8 issue of the journal Nature, in a paper headlined by graduate student Nicholas McConnell and professor Chung-Pei Ma of The University of California, Berkeley.

The team measured the black holes' masses by combining observations of the fast-moving stars at their hearts made with the giant Gemini and Keck telescopes in Hawaii with observations of their diffuse outer regions using the George and Cynthia Mitchell Spectrograph on the 2.7-meter Harlan J. Smith Telescope at The University of Texas at Austin's McDonald Observatory.

Gebhardt and Murphy have been leading an effort to exploit the powerful Mitchell Spectrograph to study these galaxies. Earlier this year, they announced the previous record-holder black hole mass in the galaxy M87, at 6.7 billion suns.

"They just keep getting bigger," Gebhardt said. "Our measured black hole in M87 is now dwarfed by about a factor of two by both of these. It's all very exciting, since it tells us something fundamental about how galaxies form."

Putting together the Gemini and Keck observations of the smallest, innermost parts of these galaxies with the McDonald observations of their largest, outmost regions to deduce the masses of their central black holes was a big challenge.

"We needed computer simulations that can accommodate such huge changes in scale," Gebhardt said. "This can only be done on a supercomputer." The team used the supercomputers at the Texas Advanced Computing Center (TACC) of The University of Texas at Austin.

The black holes in question are at the centers of two galaxies more than 300 million light years from Earth, and may be the dark remnants of some of the very bright galaxies, called quasars, that populated the early universe.

“In the early universe there were lots of quasars or active galactic nuclei, and some were expected to be powered by black holes as big as 10 billion solar masses or more. But if they were so massive in their youth, where are they today?” Ma said. “These two new supermassive black holes are similar in mass to young quasars, and may be the missing link between quasars and the supermassive black holes we see today.”

Black holes are dense concentrations of matter that produce such strong gravitational fields that even light cannot escape. While exploding stars, called supernovae, can leave behind black holes the mass of a single star, supermassive black holes have presumably grown from the merger of other black holes or by capturing huge numbers of stars and massive amounts of gas.

“These black holes may shed light on how black holes and their surrounding galaxies have nurtured each other since the early universe,” McConnell said.

One of the newly discovered black holes is 9.7 billion solar masses and is located in the elliptical galaxy NGC 3842, which is the brightest galaxy in the Leo cluster of galaxies that sits 320 million light years away in the direction of the constellation Leo. The second is as large or larger and sits in the elliptical galaxy NGC 4889, which is the brightest galaxy in the Coma cluster about 336 million light years from Earth in the direction of the constellation Coma Berenices.

According to McConnell, these black holes have an event horizon — the “abandon all hope” edge from which not even light can escape — 200 times the orbit of Earth, or five times the orbit of Pluto. Beyond the event horizon, each black hole has a gravitational influence that would extend over a sphere 4,000 light years across.

“For comparison, these black holes are 2,500 times as massive as the black hole at the center of the Milky Way Galaxy, whose event horizon is one fifth the orbit of Mercury,” McConnell said.

These 10 billion solar mass black holes have remained hidden until now presumably because they are living in quiet retirement, Ma said. During their active quasar days some 10 billion years ago they cleared out the neighborhood by swallowing vast quantities of gas and dust. The surviving gas became stars that have since orbited peacefully. According to Ma, these monster black holes and their equally monster galaxies containing probably a trillion stars settled into obscurity at the center of galaxy clusters.

Astronomers believe that many if not all galaxies have a massive black hole at the center, with the larger galaxies harboring larger black holes. The largest black holes are found in elliptical galaxies, thought to result from the merger of two spiral galaxies. Ma found, however, that mergers of elliptical galaxies themselves could produce the largest elliptical galaxies and supermassive black holes approaching 10 billion solar masses. These black holes can grow even larger by consuming gas left over after a merger.

“Multiple mergers are one way to build up these behemoths,” she said.

“For an astronomer, finding these insatiable black holes is like finally encountering people nine feet tall, whose great height had only been inferred from fossilized bones. How did they grow so large?” Ma said. “This rare find will help us understand whether these black holes had very tall parents or ate a lot of spinach.”

Other coauthors of the Nature paper are James Graham, a professor of astronomy at UC Berkeley and The University of Toronto, postdoctoral fellow Shelley A. Wright at UC Berkeley, Tod R. Lauer of the National Optical Astronomy Observatory, and Douglas O. Richstone of the University of Michigan.

The research was supported by the National Science Foundation, the National Aeronautics and Space Administration and UC Berkeley’s Miller Institute for Basic Research in Science.

Science contacts:

Karl Gebhardt
Herman and Joan Suit Professor of Astrophysics
The University of Texas at Austin
512-590-5206 (mobile)

Jeremy Murphy
The University of Texas at Austin

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