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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.
"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.
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
Credit: NASA/AURA/STScI and WikiSky/SDSS
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
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
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
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.
The Astrophysical Journal Letters, Volume 691, Issue 2, pp. L142-L146 (2009).
The University of Texas at Austin
Dr. Mona Clerico
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone: +49 89 30000-3980
Prof. Dr. Ralf Bender
Max-Planck-Institut für extraterrestrische Physik
Phone: +49 89 30000-3503
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