Scientists at the Carnegie Observatories in Pasadena, California, have
uncovered six times the expected number of active, supermassive black holes
in a single viewing of a cluster of galaxies, a finding that has profound
implications for theories as to how old galaxies fuel the growth of their
central black holes.

The finding suggests that voracious, central black holes might be as common
in old, red galaxies as they are in younger, blue galaxies, a surprise to
many astronomers. The team made this discovery with NASA’s Chandra X-ray
Observatory. They also used Carnegie’s 6.5-meter Walter Baade Telescope
at the Las Campanas Observatory in Chile for follow-up optical observations.

“This changes our view of galaxy clusters as the retirement homes for old and
quiet black holes,” said Dr. Paul Martini, lead author on a paper describing
the results that appears in the September 10 issue of The Astrophysical
Journal Letters. “The question now is, how do these black holes produce
bright X-ray sources, similar to what we see from much younger galaxies?”

Typical of the black hole phenomenon, the cores of these active galaxies are
luminous in
X-ray radiation. Yet, they are obscured, and thus essentially undetectable in
the radio, infrared and optical wavebands.

“X rays can penetrate obscuring gas and dust as easily as they penetrate the
soft tissue of the human body to look for broken bones,” said co-author Dr.
Dan Kelson. “So, with
Chandra, we can peer through the dust and we have found that even ancient
galaxies with 10-billion-year-old stars can have central black holes still
actively pulling in copious amounts of interstellar gas. This activity has
simply been hidden from us all this time. This means these galaxies aren’t
over the hill after all and our theories need to be revised.”

Scientists say that supermassive black holes — having the mass of millions
to billions of suns squeezed into a region about the size of our Solar System
— are the engines in the cores of bright active galaxies, often referred to
as Active Galactic Nuclei, or AGN.
Many astronomers think that all galaxies have central, supermassive black
holes, yet only a small percent show activity. What is needed to power the
AGN is fuel in the form of a nearby reservoir of gas and dust.

Galaxy clusters contain hundreds to thousands of galaxies. They are the
largest known structures in the Universe and serve as a microcosm for the
mechanics of the Universe at large. The galaxies in clusters are often old,
reddish elliptically shaped galaxies, distinct from blue, spiral galaxies
like our own. These old galaxies also do not have many young stars.

The theory now in question is that as galaxies enter into clusters at high
speeds, they are stripped of their interstellar gas, much as a strong wind
strips leaves from a tree. Galaxies may also collide with one another and use
up all of their gas in one huge burst of star formation triggered by this
interaction. These processes remove most, if not all, of the gas that isn’t
locked up in stars. As they no longer have the raw material to form new
stars, the stellar population slowly gets old and the Galaxy appears red. No
gas is left to fuel an AGN.

Previous surveys of galaxy clusters with optical telescopes have found that
about only one percent of the galaxies in a cluster have AGN. This latest
Chandra observation if typical, however, bumps the count up to about 5
percent. The team found six red galaxies with high x-ray activity during a
nearly 14-hour Chandra observation of a galaxy cluster named Abell 2104, over
700 million light years from Earth. Based on previous optical surveys, only
one was expected.

“If we relied on optical data alone, we would have missed these hidden
monsters,” said co-author Dr. John Mulchaey. Only one of the six AGN, in
fact, had the optical spectral properties typical of AGN activity.

“The presence of these AGN indicate that supermassive black holes have
somehow retained a fuel source, despite the harsh treatment galaxies suffer
in clusters, and are now coming out of retirement,” said Martini.

This could imply that galaxies are better at holding onto a supply of gas and
dust than previously thought, particularly deep down at their cores near the
supermassive black hole. This gas and dust may also be the same material that
obscures the AGN at other wavelengths.

The presence of so many AGN could also contribute to the radio and infrared
radiation from the clusters, which until now was thought to be almost
exclusively a product of star formation. Thus, scientists may be
overestimating the amount of star formation taking place in clusters.

The Carnegie group has begun a study of other galaxy clusters with Chandra.
Martini and Kelson are postdoctoral researchers at the Carnegie Observatories
in Pasadena; Mulchaey is a staff astronomer. NASA’s Marshall Space Flight
Center in Huntsville, Ala., manages the Chandra program, and TRW, Inc.,
Redondo Beach, Calif., is the prime contractor. The Smithsonian’s Chandra
X-ray Center controls science and flight operations from Cambridge, Mass.

Images and additional information about this result are available at: