A New Way to Weigh Giant Black Holes (Forwarded)

Jennifer Morcone
Marshall Space Flight Center, Huntsville, Ala. July 16, 2008
256-544-0034

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998

News release: 08-086

A New Way to Weigh Giant Black Holes

How do you weigh the biggest black holes in the universe? One answer now
comes from a completely new and independent technique that astronomers have
developed using data from NASA's Chandra X-ray Observatory.

By measuring a peak in the temperature of hot gas in the center of the giant
elliptical galaxy NGC 4649, scientists have determined the mass of the
galaxy's supermassive black hole. The method, applied for the first time,
gives results that are consistent with a traditional technique.

Astronomers have been seeking out different, independent ways of precisely
weighing the largest supermassive black holes, that is, those that are
billions of times more massive than the Sun. Until now, only methods based
on observations of the motions of stars or of gas in a disk near such large
black holes had been used.

"This is tremendously important work since black holes can be elusive, and
there are only a couple of ways to weigh them accurately," said Philip
Humphrey of the University of California at Irvine, who led the study. "It's
reassuring that two very different ways to measure the mass of a big black
hole give such similar answers."

NGC 4649 is now one of only a handful of galaxies for which the mass of a
supermassive black hole has been measured with two different methods. In
addition, this new X-ray technique confirms that the supermassive black hole
in NGC 4649 is one of the largest in the local universe with a mass about
3.4 billion times that of the Sun, about a thousand times bigger than the
black hole at the center of our galaxy.

The new technique takes advantage of the gravitational influence the black
hole has on the hot gas near the center of the galaxy. As gas slowly settles
towards the black hole, it gets compressed and heated. This causes a peak in
the temperature of the gas right near the center of the galaxy. The more
massive the black hole, the bigger the temperature peak detected by Chandra.

This effect was predicted by two of the co-authors -- Fabrizio Brighenti
from the University of Bologna, Italy, and William Mathews from the
University of California at Santa Cruz -- almost 10 years ago, but this is
the first time it has been seen and used.

"It was wonderful to finally see convincing evidence of the effects of the
huge black hole that we expected," said Brighenti. "We were thrilled that
our new technique worked just as well as the more traditional approach for
weighing the black hole."

The black hole in NGC 4649 is in a state where it does not appear to be
rapidly pulling in material towards its event horizon or generating copious
amounts of light as it grows. So, the presence and mass of the central black
hole has to be studied more indirectly by tracking its effects on stars and
gas surrounding it. This technique is well suited to black holes in this
condition.

"Monster black holes like this one power spectacular light shows in the
distant, early universe, but not in the local universe," said Humphrey. "So,
we can't wait to apply our new method to other nearby galaxies harboring
such inconspicuous black holes."

These results will appear in an upcoming issue of The Astrophysical Journal.
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/2008/ngc4649/
and
http://chandra.nasa.gov