A New Way To Weigh
Giant Black Holes
(16 July 2008) 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 centre 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,
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."
AnimationChandra X-ray Image
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 centre of our galaxy.
The new
technique takes advantage of the gravitational influence the black hole has on
the hot gas near the centre 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 centre 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, nor 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
harbouring 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.
(source: NASA Marshall Space Flight Center)
