Pasadena, California. Until now astronomers thought that old, red cluster
galaxies were past their prime and subdued. Only about one percent were
supposed to have Active Galactic Nuclei (AGN)–violent centers where
supermassive black holes gobble up surrounding material and emit it as
X-rays. A surprising find by a team of astronomers at the Carnegie
Observatories in Pasadena, California, has altered this belief. Using a
combination of space-based X-ray and Earth-based optical instrumentation,
the scientists found that six times the expected number of galaxies in a
nearby cluster have active centers. “This changes our view of galaxy
clusters as the retirement homes for old and quiet black holes,” said team
member, Dr. Dan Kelson. “The question now is: How do these black holes turn
themselves on again?” The discovery has also brought into question how
galaxies evolve and how stars form in these environments.

The Carnegie group, headed by Dr. Paul Martini, published their results in
the September 10, 2002, Astrophysical Journal Letters. They took an unusual
approach to their study by using NASA’s X-ray Chandra satellite in concert
with Carnegie’s new 6.5-meter Walter Baade optical telescope at the Las
Campanas observatory in Chile. Using the Chandra data, they discovered six
X-ray sources in galaxy cluster Abell 2104, about 700 million light-years
from Earth. They then used the Carnegie telescope to confirm that all of
the galaxies are in the cluster and not in the foreground or background.
“If we used optical data alone, we would have missed these hidden
monsters,” said Dr. John Mulchaey. “If we’d used only X-ray data, we would
not have been sure that all the AGNs were in the cluster,” he concluded.

Galaxy clusters typically have hundreds to thousands of galaxy members. The
Carnegie researchers surveyed the 100 brightest galaxies in Abell 2104. It
is believed that old, red galaxies generally populate clusters because
during cluster formation the raw material for making stars and feeding
black holes–gas–is burned off and nothing is left to fuel these
systems. “The presence of these AGNs indicate that supermassive black
holes have somehow retained a fuel source,” said Martini. “Despite the
harsh treatment these galaxies suffered as a cluster, they seem to be
having the black hole equivalent of a mid-life crisis. They aren’t over the
hill after all.” The Carnegie group has already started studying other
clusters to see if similar activity is present elsewhere.

Image available at

Figure caption: An X-ray picture (false color) of the Abell 2104 cluster of
galaxies taken with NASA’s Chandra X-ray Observatory overlaid on an optical
image taken with Carnegie Observatories’ 6.5-meter Walter Baade telescope
in Las Campanas, Chile. The Chandra image reveals X-ray emissions produced
by both hot gas (the blue area near the center of the image) and by
accretion of dust and gas onto supermassive black holes (the smaller blue
patches on the outskirts of the image). The number of active, supermassive
black holes found in this cluster is six times the amount found using other
techniques.The finding suggests that active black holes are much more
common in clusters of galaxies than previously believed. (Image courtesy
NASA and the Carnegie Observatories.)