Observed Across the Spectrum, a Black Hole Reveals its Secrets
Tuesday, October 9, 2007
Science Update - A look at CfA discoveries from recent journals

Astronomers think that most galaxies, including our own Milky Way, harbor massive black holes at their centers. These black holes can contain billions of solar masses of material, and are thought to develop during collisions between galaxies. Astronomers observing galaxies sometimes see clear evidence of a massive black hole through its dramatic activity -- powerful jets of material streaming outward from its vicinity as material accretes onto a surrounding disk, for example. But often astronomers see very little going on -- the Milky Way's center is such a quiescent case. Massive black holes help to determine the future behavior of the galaxy via their powerful gravitational force, and when they are accompanied by outflows or starbursts, they light up the galaxy and further influence its evolution. Massive black holes are therefore essential players in a galaxy's life, yet astronomers do not understand how they initially develop, or how they affect the subsequent evolution of a galaxy and, in particular, the formation of its stars.

A team of seven SAO astronomers, Aneta Siemiginowska, Giuseppina Fabbiano, Martin Elvis, Lincoln Greenhill, Roberto Soria, Alessandro Baldi, and Dong-Woo Kim, together with a colleague, have combined Chandra X-ray Observatory data with infrared observations from the Spitzer Space Telescope, optical results from Hubble, and radio data from the Very Large Array to unravel some of the mystery. They measured the light from an isolated galaxy that lies about 75 million light-years away. It had been assumed without much evidence that this galaxy contained a massive black hole that was quiescent, but recently very sensitive Chandra observations found evidence for X-ray emission signifying that some energetic process was indeed underway at the nucleus, evidence for possible modest accretion onto a disk surrounding a black hole; other possibilities include very hot gas in the region, or X-ray emitting binary stars. By taking advantage of information that spanned the spectrum, the team was able to conclude that star formation is not happening there, nor is there much hot gas, and that X-ray binaries present cannot be the nuclear source seen. The scientists conclude that, although further detailed studies are warranted, even apparently "dead" massive black holes in nearby galaxies can retain some degree of activity. The result is in agreement with computer simulations of aged galactic nuclei, and helps to confirm the general picture of the role of black holes in the life of a galaxy.