J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
WASHINGTON -- Astronomers have found an
extraordinary galaxy cluster, one of the largest objects in the universe, that
is breaking several important cosmic records. Observations of the Phoenix
cluster with NASA's Chandra X-ray Observatory, the National Science
Foundation's South Pole Telescope, and eight other world-class observatories
may force astronomers to rethink how these colossal structures and the galaxies
that inhabit them evolve.
Stars are forming in the Phoenix cluster
at the highest rate ever observed for the middle of a galaxy cluster. The
object also is the most powerful producer of X-rays of any known cluster and
among the most massive. The data also suggest the rate of hot gas cooling in
the central regions of the cluster is the largest ever observed.
The Phoenix cluster is located about 5.7
billion light years from Earth. It is named not only for the constellation in
which it is located, but also for its remarkable properties.
"While galaxies at the center of
most clusters may have been dormant for billions of years, the central galaxy
in this cluster seems to have come back to life with a new burst of star
formation," said Michael McDonald, a Hubble Fellow at the Massachusetts
Institute of Technology and the lead author of a paper appearing in the Aug. 16
issue of the journal Nature. "The mythology of the Phoenix, a bird rising
from the dead, is a great way to describe this revived object."
Like other galaxy clusters, Phoenix
contains a vast reservoir of hot gas, which itself holds more normal matter --
not dark matter -- than all of the galaxies in the cluster combined. This
reservoir can be detected only with X-ray telescopes such as Chandra. The
prevailing wisdom once had been that this hot gas should cool over time and
sink to the galaxy at the center of the cluster, forming huge numbers of stars.
However, most galaxy clusters have formed very few stars during the last few
billion years. Astronomers think the supermassive black hole in the central
galaxy of a cluster pumps energy into the system, preventing cooling of gas
from causing a burst of star formation.
The famous Perseus cluster is an example
of a black hole bellowing out energy and preventing the gas from cooling to
form stars at a high rate. Repeated outbursts in the form of powerful jets from
the black hole in the center of Perseus created giant cavities and produced sound
waves with an incredibly deep B-flat note 57 octaves below middle C, which, in
turn, keeps the gas hot.
"We thought that these very deep
sounds might be found in galaxy clusters everywhere," said co-author Ryan
Foley, a Clay Fellow at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Mass. "The Phoenix cluster is showing us this is not the case
-- or at least there are times the music essentially stops. Jets from the giant
black hole at the center of a cluster are apparently not powerful enough to
prevent the cluster gas from cooling."
With its black hole not producing
powerful enough jets, the center of the Phoenix cluster is buzzing with stars
that are forming about 20 times faster than in the Perseus cluster. This rate
is the highest seen in the center of a galaxy cluster but not the highest seen
anywhere in the universe. However, other areas with the highest star formation
rates, located outside clusters, have rates only about twice as high.
The frenetic pace of star birth and
cooling of gas in the Phoenix cluster are causing the galaxy and the black hole
to add mass very quickly -- an important phase the researchers predict will be
relatively short-lived.
"The galaxy and its black hole are undergoing
unsustainable growth," said co-author Bradford Benson, of the University
of Chicago. "This growth spurt can't last longer than about a hundred
million years. Otherwise, the galaxy and black hole would become much bigger
than their counterparts in the nearby universe."
Remarkably, the Phoenix cluster and its
central galaxy and supermassive black hole are already among the most massive
known objects of their type. Because of their tremendous size, galaxy clusters
are crucial objects for studying cosmology and galaxy evolution, so finding one
with such extreme properties like the Phoenix cluster is important.
"This spectacular star burst is a
very significant discovery because it suggests we have to rethink how the
massive galaxies in the centers of clusters grow," said Martin Rees of
Cambridge University, a world-renowned expert on cosmology who was not involved
with the study. "The cooling of hot gas might be a much more important
source of stars than previously thought."
The Phoenix cluster originally was
detected by the National Science Foundation's South Pole Telescope, and later
was observed in optical light by the Gemini Observatory, the Blanco 4-meter
telescope and Magellan telescope, all in Chile. The hot gas and its rate of
cooling were estimated from Chandra data. To measure the star formation rate in
the Phoenix cluster, several space-based telescopes were used, including NASA's
Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer and ESA's
Herschel.
NASA's Marshall Space Flight Center in
Huntsville, Ala., manages the Chandra Program for NASA's Science Mission
Directorate in Washington. The Smithsonian Astrophysical Observatory controls
Chandra's science and flight operations from Cambridge, Mass.
For Chandra images, multimedia and
related materials, visit http://www.nasa.gov/chandra.
For an additional interactive image,
podcast, and video on the finding, visit http://chandra.si.edu.
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