David Aguilar
(617) 495-7462

Christine Pulliam
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CfA Press Release

FOR RELEASE: 10:20 a.m. EDT April 13, 1999


Astronomers have heard the first shy words from seemingly quietsupermassive black holes in the form of a unique type of X-raylight. These black holes exist in the centers of the oldest, largestgalaxies and have a mass of about a billion suns, compressed into aregion comparable to the size of our Solar System.

While a small percentage of supermassive black holes generate hugeX-ray glows (in a phenomenon known as an active galactic nucleus,or AGN) the vast majority of supermassive black holes are quiescent,producing no detectable X-rays. They are known only through theirgravitational effects on star and gas rotation.

These new observations suggest that quiescent supermassive blackholes do produce X-rays, but the emission is much feebler than thatobserved in AGN and creates a different kind of light spectrum.The new results add credence to the ubiquity of supermassive black holes,which are likely to exist in the cores of all galaxies, including ourown. Also, the unique X-ray light may provide clues to theorigin of Universe's still unexplained X-ray background radiation.

Dr. Tiziana Di Matteo of the Harvard-Smithsonian Center forAstrophysics in Cambridge, Mass., and Dr. Steven W. Allen of theInstitute of Astronomy in Cambridge, UK, present these results at theHigh Energy Astrophysics Division of the American Astronomical Societymeeting in Charleston, S.C., April 13, 1999.

"We have found that these giant black holes, lurking at the centers ofnearby galaxies, are not totally hidden from us, but manifestthemselves by radiating small amounts of very energetic X-ray light,"said Dr. Di Matteo. "We cannot explain this light as being due to anyother type of source."

Dr. Allen said the key evidence was the type of light: energetic X-raylight with shorter wavelengths. The X-ray light that Drs. Di Matteoand Allen observed in six nearby old galaxies was relatively moreenergetic compared to light associated with very luminous black holesin AGN and quasars, albeit in smaller quantities.

The differences in light characteristics, Dr. Di Matteo said, might bedue to the different ways in which matter falls into thesesupermassive black holes.

Matter does not fall directly into a black hole, but rather spirals in-- like water down a drain -- through what is known as anaccretiondisk. In younger, spiral-shaped galaxies, matter accumulates into adense disk;particles collide frequently and radiate X-rays, keeping thetemperature of the disk at millions of degrees and the disk relativelythin. Such is the case for AGN.

In older, elliptical galaxies, suchas the six that Drs. Di Matteo and Allen studied, the black holeaccretion disk accumulates matter at a slower rate and collisionsare much less frequent, causing the disk to be tenuous and bloated and the matter in the disk to grow very hot. The result is amuch less luminous black hole region that converts the availableenergy into radiation much less efficiently.

"This is a wonderful solution for the problem of dimblack holes," said Dr. Di Matteo."The spectrum of radiation, ranging from high energyX-rays to radio waves, emitted from the centers of these sixelliptical galaxies, matches that predicted by models of suchlow-efficiency accretion."

The astronomers note that it is very interesting that this"low-efficiency" mode of accretion is occurring in older galaxies.The abundance of newborn galaxies in the early Universe, they said,would have provided black holes with plenty of gas to feed on, enoughto generate the light of quasars. As these galaxies matured, theyconverted their gas into stars, limiting the fuel available to thecentral black holes. With less fuel to feed their black holes,low-efficiency accretion takes over. Today, the cores of galaxies thatwere once ablaze with quasar light find themselves in the dark ages.Drs. Di Matteo and Allen's results further corroborate theidea that the present-day Universe may therefore be rife with deadquasars.

The astronomers were enthusiastic about the launch later thisyear of NASA's Chandra Observatory, a revolutionary X-rayastronomy satellite, which astronomers hope will help provide clearanswers tothe nature and origin of the X-ray light from supermassive black holesin the centers of galaxies.

Drs. Di Matteo and Allen utilized data collected by the AdvancedSatellite for Cosmology and Astrophysics (ASCA), a Japanese X-raysatellite launched in 1993 with a Japanese and NASA payload. Dr. DiMatteo is a Chandra Postdoctoral Fellow in the Theory Group at the Harvard-Smithsonian Centerfor Astrophysics. Dr. Allen is part of the High Energy AstrophysicsGroup at the Institute of Astronomy, University of Cambridge.

The research team also included Prof. Andrew C. Fabian of the Institute ofAstronomy, Cambridge, Eliot Quataert, and Prof. Ramesh Narayan of the Center for Astrophysics.

Click here for images and captions accompanying this release.

For more information, contact:

Tiziana Di Matteo, 617/496-3320, tdimatteo@cfa.harvard.edu
Steve Allen, +44/1223/330803, swa@ast.cam.ac.uk

Public Affairs

Megan Watzke, CfA, 617/495-7463, mwatzke@cfa.harvard.edu
William Steigerwald, GSFC, 301/286-5017, William.A.Steigerwald.1@gsfc.nasa.gov

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