Chandra weighs up supermassive black hole
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 17, 2008

Until now, the mass of a black hole has been assessed by studying the motions of proximal stars and gas, but thanks to an innovative new technique, these measurements can now also be derived from the temperatures of hot gasses compressed in the jaws of these celestial cannibals.

Over ten years ago, two scientists – Fabrizio Brighenti from the University of Bologna, Italy, and William Mathews from the University of California at Santa Cruz – predicted that the gravitational influence that supermassive black holes exert on their constituent gases as they settle towards the centre of the black hole would be sufficient to produce a peak temperature that would be detectable in X-ray observations. The more massive the black hole, the bigger the observed temperature peak would be.

NGC 4649 is located 51 million light years away from Earth and contains one of the local Universe’s biggest black holes, yet the lack of a bright central point in either the Chandra X-ray (left) or Hubble optical (right) images shows that the black hole does not appear to be pulling in material or generating huge amounts of light. By looking at the temperatures in the centre of the black hole, scientists have come up with a new way of measuring the masses of seemingly dormant black holes. Images: NASA/CXC/Univ. of California Irvine/P.Humphrey et al. (X-ray) and NASA/STScI (optical).

Now, in a study of elliptical galaxy NGC 4649, which contains one of the biggest black holes in the local Universe, and aided by Chandra X-ray observations, the scientists were finally able to put their theory into practice. Their derivation of the black hole’s mass of 3.4 billion times the mass of the Sun matched the results derived by the more traditional method which is based on observations of the motions of stars or gas in a disc surrounding the black hole.

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

Because NGC 4649’s black hole shows no overt signs of its presence – there does not appear to be rapid influx of material careering towards its centre, or copious amounts of light generated as it grows – it lends itself to analysis by this new technique.

"This is tremendously important work since black holes can be elusive, and there are only a couple of ways to weigh them accurately," says 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 one of only a handful of galaxies for which the mass of a supermassive black hole has been measured with two different methods, and the research team are champing at the bit to apply their new technique to other nearby galaxies that could also be harbouring such inconspicuous black holes.