In the last decade or so, supermassive black holes at the centers of galaxies have become an accepted part of the astronomical landscape, as have the small, stellar-mass black holes found farther from the center. Missing has been any solid evidence of an intermediate mass black hole, an entirely new astronomical object that would be, according to University of Michigan astronomer Joel Bregman, "the missing link in the black hole chain."

An X-ray image taken by Chandra shows a very bright point source in a spiral arm of the galaxy Messier 81, shown as the yellow box on this optical color image. Credit: University of Michigan

Intermediate mass black holes may produce the brilliant X-ray light seen from sources in the disks of galaxies and finding their optical counterparts is a crucial step. Bregman and his colleagues, Jifeng Liu and Patrick Seitzer, also at the U-M, have found such optical counterparts and they presented their report this week to the American Astronomical Society meeting in Washington, D.C.

Like many other astronomical bodies, black holes can only be studied indirectly, through the force they exert on nearby objects and from the light given off by material before it falls into the hole. X-rays are particularly useful in locating binary systems such as black holes with companion stars so close that they are slowly being eaten by the black hole. As the two objects orbit around each other the black hole pulls gas off the surface of the nearby star and this material spins around the black hole, spiraling in toward the center and emitting X-rays. The maximum brightness of the X-ray light is proportional to the black hole mass.

The Michigan researchers' discovery centers on the enigmatic Ultraluminous X-ray sources (ULX), which have been causing great excitement in the high energy community for several years. Various theories have been devised to explain these objects, which have luminosities 10 times greater than the brightest X-ray point sources in the Milky Way. The high X-ray luminosity suggests a black hole mass at least 30 to 1,000 times the mass of the sun. This would make them 10-100 times larger than stellar black holes, but a million times smaller than the supermassive black holes found at the center of galaxies.

"For years, these mysterious X-ray sources had no optical counterpart, so we were not positive that they were binary star systems," said Bregman, "but by combining the power of the Chandra X-Ray Observatory and the Hubble Space Telescope, we thought that we could find the star that is being eaten." The researchers devised a method for overlaying the high-resolution X-ray images from the Chandra X-Ray Observatory on top of the optical imagery from the Hubble Space Telescope (HST). "Overlaying these two images to very high precision was a technical challenge," said. Liu. This problem was overcome by aligning the two images by objects common to both, such as background Active Galactic Nuclei.

Bregman notes that selecting the right targets can also be tricky, but eventually the team came up with five good ones, two in M51 and one each in M81, NGC6946, and NGC4559. "Each of the targets appears to be close to spiral arms, which is already an exciting result. It suggests that they are associated with young high-mass stars," he said.

The Chandra data have already been obtained for M51 and M81, and optical counterparts to the ULXs have been identified. "For the first time, we've identified a star that's being eaten," Bregman said. "This companion star has blue colors, which indicates that it is a massive young star, rather than an old star like the sun," said Seitzer. Building on this success, Bregman said that "our next step is to measure how fast the star and the black hole are orbiting each other. If we can do that, we can measure the mass of the black hole from the mass of the companion star and the orbital period."

"Either we will discover the missing link in the black hole chain or we will find a more modest black hole that has somehow beaten all our standard rules about how brightly it can shine. Only time -- and more observations -- will tell," Bregman said.

The work was conducted through the Chandra and Hubble Guest Observer Programs and is funded by the National Aeronautics and Space Administration.