Max-Planck-Institut
  für extraterrestrische Physik

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Astronomers Discover Link Between Supermassive Black Holes and Galaxy Formation

A pair of astronomers from Texas and Germany have used a telescope at The University of Texas at Austin's McDonald Observatory together with the Hubble Space Telescope and many other telescopes around the world to uncover new evidence that the largest, most massive galaxies in the universe and the supermassive black holes at their hearts grew together over time.

Two giant elliptical galaxies, NGC 4621 and NGC 4472, look similar from a distance, as seen on the right in images from the Sloan Digital Sky Survey. But zooming into these galaxies' cores with Hubble Space Telescope reveals their differences (left, black and white images). NGC 4621 shows a bright core, while NGC 4472 is much dimmer. The core of this galaxy is populated with fewer stars. Many stars have been slung out of the core when the galaxy collided and merged with another. Their two supermassive black holes orbited each other, and their great gravity sent stars careening out of the galaxy's core. Credit: NASA/AURA/STScI and WikiSky/SDSS

"They evolved in lockstep," said The University of Texas at Austin's John Kormendy, who co-authored the research that appears in Astrophysical Journal Letters with Ralf Bender of Germany's Max-Planck-Institute for Extraterrestrial Physics and Ludwig Maximilians University Observatory.

Astronomers know that galaxies, those vast cities of millions or billions of stars, grow larger through collisions and mergers. Kormendy and Bender's work involves the biggest galaxies in the universe - "elliptical galaxies" that are shaped roughly like footballs and that can be made of as many as a thousand billion stars. Virtually all of these galaxies contain a black hole at their centers, that is, an infinitely dense region which contains the mass of millions or billions of Suns and from which no light can escape.

A current leading theory says that when galaxies collide, their black holes end up revolving around each other. Together, the two black holes act like an egg beater: they violently stir up the galaxy center with their incredibly strong gravity, and they fling stars out of the central regions. As the black hole pair sinks to the center of the new merger remnant, this supergalaxy's core is depleted of the stars that were flung away. Kormendy and Bender measured the resulting light deficits.

Light deficits in galaxy cores are surprising in view of decades of work by many astronomers, including Kormendy and Bender, which showed that the biggest elliptical galaxies contain the most massive black holes at their centers. These are monsters "weighing in" at a billion or more times the mass of our Sun. They attract the stars around them with ferociously strong gravity. Astronomers expected that such big black hole would yank the galaxy's stars into a tiny, dense cluster at the center. But observations in the 1980s with ground-based telescopes and much better observations in the 1990s with the Hubble Space Telescope revealed the opposite. The biggest galaxies have big, fluffy, low-density centers. Why are giant black holes not surrounded by dense cluster of stars? Where did the missing stars go?

The theory that black hole binaries gravitationally slingshot the stars out of galactic centers has been the popular but unproved explanation. No telescope observations provided compelling evidence - until now.

"Our new observations are a strong and direct link between black holes and galaxy central properties," Kormendy says. "They are a 'smoking gun' that connects black holes with the formation of the surprisingly fluffy centers of giant elliptical galaxies."

Kormendy and Bender made detailed studies of 11 such galaxies in the Virgo Cluster. To get a comprehensive overall picture of each galaxy, they used the wide field of view of the Prime Focus Camera on McDonald Observatory's 0.8-meter Telescope. They used Hubble Space Telescope to study these same galaxies' cores in great detail. Many other telescopes were used to connect the central data from Hubble with the outer data from the McDonald telescope. The results on 27 Virgo ellipticals measured by Kormendy, Bender, and their University of Texas colleagues David Fisher and Mark Cornell are scheduled for publication in a forthcoming issue of the Astrophysical Journal Supplements.

Their precision measurements of the brightnesses - that is, the number of stars - at various distances from the centers of elliptical galaxies allowed them to calculate much more accurately than previously the masses of stars that are "missing" in the centers of the biggest ellipticals. This revealed more surprises: The missing mass increases in lockstep with the measured masses of the central black holes. It was known that the two quantities are related, but it was not known that the correlation is so tight as to be within the margin of error. That is, the correlation is virtually perfect.

The missing mass also increases in lockstep with another galaxy property that is known to be tied directly to black holes, namely the speeds at which stars move far out in the galaxy where they cannot feel the black hole's gravity.

"Astronomers love tight correlations," Bender says. "They tell us what is connected with what. The new observations give us much stronger evidence that black holes control galaxy formation, at least at their centers."

Finally, Kormendy adds: "Measurements of the missing mass and measurements of the speeds at which stars move in elliptical galaxies now give us two independent ways to estimate black hole masses. Comparing them with each other and with direct mass measurements gives us a better understanding of how galaxies and their black holes grew up together."

A joint news release by The University of Texas at Austin McDonald Observatory,
the Max-Planck-Institute for Extraterrestrial Physics (Munich, Germany),
the Observatory of the Ludwig Maximilians University (Munich, Germany),
and the U. S. National Science Foundation.

Original paper:

      The Astrophysical Journal Letters, Volume 691, Issue 2, pp. L142-L146 (2009).


Contact:

Rebecca Johnson
McDonald Observatory
The University of Texas at Austin
Phone: 512-475-6763 E-mail: rjohnson@astro.as.utexas.edu

Dr. Mona Clerico
Press Officer
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone: +49 89 30000-3980
E-Mail: clerico@mpe.mpg.de

Prof. Dr. Ralf Bender
Director
Max-Planck-Institut für extraterrestrische Physik
Phone: +49 89 30000-3503
E-Mail: bender@mpe.mpg.de



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