Chandra Catches "Piranha" Black Holes (Forwarded)

Jennifer Morcone
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256/544-7199)

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
(Phone: 617/496-7998)

CXC Release

For Release: July 24, 2007

Chandra Catches "Piranha" Black Holes

Supermassive black holes have been discovered to grow more rapidly in young
galaxy clusters, according to new results from NASA's Chandra X-ray
Observatory. These "fast-track" supermassive black holes can have a big
influence on the galaxies and clusters that they live in.

Using Chandra, scientists surveyed a sample of clusters and counted the
fraction of galaxies with rapidly growing supermassive black holes, known as
active galactic nuclei (or AGN). The data show, for the first time, that
younger, more distant galaxy clusters contained far more AGN than older,
nearby ones.

Galaxy clusters are some of the largest structures in the Universe,
consisting of many individual galaxies, a few of which contain AGN. Earlier
in the history of the universe, these galaxies contained a lot more gas for
star formation and black hole growth than galaxies in clusters do today.
This fuel allows the young cluster black holes to grow much more rapidly
than their counterparts in nearby clusters.

"The black holes in these early clusters are like piranha in a very well-fed
aquarium," said Jason Eastman of Ohio State University (OSU) and first
author of this study. "It's not that they beat out each other for food,
rather there was so much that all of the piranha were able to really thrive
and grow quickly."

The team used Chandra to determine the fraction of AGN in four different
galaxy clusters at large distances, when the Universe was about 58% of its
current age. Then they compared this value to the fraction found in more
nearby clusters, those about 82% of the Universe's current age.

The result was the more distant clusters contained about 20 times more AGN
than the less distant sample. AGN outside clusters are also more common when
the Universe is younger, but only by factors of two or three over the same
age span.

"It's been predicted that there would be fast-track black holes in clusters,
but we never had good evidence until now," said co-author Paul Martini, also
of OSU. "This can help solve a couple of mysteries about galaxy clusters."

One mystery is why there are so many blue, star-forming galaxies in young,
distant clusters and fewer in nearby, older clusters. AGN are believed to
expel or destroy cool gas in their host galaxy through powerful eruptions
from the black hole. This may stifle star formation and the blue, massive
stars will then gradually die off, leaving behind only the old, redder
stars. This process takes about a billion years or more to take place, so a
dearth of star-forming galaxies is only noticeable for older clusters.

The process that sets the temperature of the hot gas in clusters when they
form is also an open question. These new results suggest that even more AGN
may have been present when most clusters were forming about ten billion
years ago. Early heating of a cluster by large numbers of AGN can have a
significant, long-lasting effect on the structure of a cluster by "puffing
up" the gas.

"In a few nearby clusters we've seen evidence for huge eruptions generated
by supermassive black holes. But this is sedate compared to what might be
going on in younger clusters," said Eastman.

These results appeared in the July 20th issue of The Astrophysical Journal
Letters. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra program for the agency's Science Mission Directorate. The
Smithsonian Astrophysical Observatory controls science and flight operations
from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:
http://chandra.harvard.edu/photo/2007/agns/
and
http://chandra.nasa.gov