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Discovery adds a mid-sized class to ranks of black holes
Wednesday, April 14, 1999 By Byron Spice, Science Editor, Post-Gazette
Astronomers from Carnegie Mellon University and NASA have independently found evidence of a new class of black holes - far smaller than the supermassive black holes found in quasars, but larger than the stellar black holes only a few times more massive than our Sun.
You might call them the Goldilocks of black holes: not too big, not too small; not too hot, not too cool; not too hard, not too soft. Just right - to fit a category of black hole that astrophysicists had only speculated might exist.
Moreover, the Carnegie Mellon findings suggest that the black holes that exist at the centers of most galaxies begin to form early in galactic development, even as star birth is occurring at a dizzying rate.
Both groups based their discoveries on X-ray observations and announced their findings yesterday at a Charleston, S.C., meeting of the American Astronomical Society's High-Energy Astrophysics Division.
Black holes are dense regions of space where the force of gravity is so strong that nothing, not even light, can escape. Stellar black holes are believed to be created when a massive star dies and collapses. Supermassive black holes, equivalent in mass to millions of stars, are thought to power quasars - distant, luminous sources of radio emissions.
Andrew Ptak, a research scientist at Carnegie Mellon, has suspected that another category of black hole must exist. While working on his doctoral thesis several years ago, he came across some X-ray emissions from the galaxy M82 that just didn't make sense otherwise.
M82, located 3.6 million light-years from Earth, is a young "starburst" galaxy with a high rate of star formation. Yet an orbiting X-ray telescope, the Japan-U.S. Advanced Satellite for Cosmology and Astrophysics, or ASCA, was detecting high-energy, or "hard," X-rays from M82 that showed matter was being heated to temperatures much higher than would normally be seen in this type of galaxy.
The best explanation would be a medium-sized black hole. Because radiation cannot escape from black holes, astronomers deduce their presence and size from the last gasp of radiation emitted by gas and particles as they spiral into a black hole. This material heats up as it is pulled into the black hole, generating X-rays. The hard X-rays from M82 suggested something was getting very, very hot, but the ASCA data was too sketchy to prove that a new class of black hole was responsible, Ptak said.
After ASCA had made repeated X-ray observations of M82, Ptak and astrophysicist Richard Griffiths took another look. This time, they found that the hard X-rays were varying on a 15-hour cycle, suggesting something very hot was orbiting a black hole larger than a stellar black hole. By their calculations, this black hole might have a mass equivalent to at least 460 Suns, but be about the size of the Earth's moon.
"We still don't know that's what we're seeing," Ptak admitted, noting the ASCA data can only trace the source of the X-rays to an area the size of our solar system. It's possible that the source may actually be several stellar black holes or an undernourished massive black hole.
But this summer, NASA will launch a new X-ray telescope, called Chandra, that will be able to pinpoint the source or sources. Griffiths, a member of the science team for Chandra's main imaging camera, said data on M82 could be available by the end of the year.
The existence of a medium-sized black hole in M82 would tell astrophysicists something about the evolution of galaxies. Because M82 is a young galaxy, it would appear that black holes begin forming early on, rather than after the galaxy is well-established and mature stars flame out.
Two astronomers from NASA's Goddard Space Flight Center in Maryland also found evidence of medium-sized black holes, though by pursuing a different line of X-ray evidence. Edward Colbert and Richard Mushotzky analyzed the spectra, or colors, of invisible X-ray light from 39 relatively nearby galaxies.
Their analysis showed that the colors were different from those emitted by supermassive black holes, suggesting instead something as massive as 100 to 10,000 Suns.
Tsunefumi Mizuno of the University of Tokyo has reported results similar to the NASA findings. Takehishi Go Tsuru and colleagues at Kyoto University have found data that support Ptak and Griffiths.
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