An artist's concept of black hole. Credit: NASA/JPL

A galaxy far beyond our own Milky Way, with a monstrous, churning black hole in its center, has been observed by two optical telescopes working in unison as an interferometer. These observations reveal the finest level of detail in a galaxy ever produced at infrared wavelengths.

Two linked telescopes at the W.M. Keck Observatory on Mauna Kea, Hawaii, observed the inner regions of the galaxy NGC 4151. The Keck Interferometer combines the world's two largest optical telescopes. A paper on the findings will appear in the October 20 issue of the Astrophysical Journal Letters.

NGC 4151 is 40 million light years from Earth, far beyond the most distant object previously detected by this type of telescope system, which was about 3,000 light years from Earth. These observations marked the first time an optical/infrared interferometer detected any object outside our galaxy and were followed a few weeks later by observations of a second galaxy with the European Southern Observatory's Very Large Telescope Interferometer.

"This opens a whole new area of research on galaxies other than our own," said Dr. Rachel Akeson, an astronomer at the Michelson Science Center at the California Institute of Technology in Pasadena. The Keck Interferometer, with its 10-meter (33-foot) telescopes, has the sensitivity needed to detect objects outside our galaxy.

The Keck Interferometer gathers light waves with two telescopes, then combines the waves so they interact, or "interfere" with each other. The system transports the light to a laboratory located between them, where a beam combiner and infrared camera combine and process the light. This technique simulates a much larger, more powerful telescope. In this respect, the Keck Interferometer is equivalent to an 85-meter (279-foot) telescope.

"Interferometry provides the angular resolution, or ability to resolve fine details, to make these kinds of observations," said the interferometer system architect, Dr. Mark Colavita of NASA's Jet Propulsion Laboratory, Pasadena, Calif.

"We couldn't observe objects as faint as this galaxy before with an interferometer. Adaptive optics on 10-meter telescopes provides the sensitivity to make this observation," said Dr. Peter Wizinowich, interferometer team lead for the W.M. Keck Observatory at the California Association for Research in Astronomy, Kamuela, Hawaii.

NGC 4151, well studied with telescopes and instruments at many wavelengths, is believed to have a black hole at its center surrounded by a doughnut-shaped ring of dust. The black hole is estimated to be 10 million times as massive as our Sun, and 10 times more massive than the black hole at the center of our own galaxy, the Milky Way. Like all black holes, its gravitational pull is so powerful that nothing, not even light, can escape. However, as it gobbles up nearby material, a fraction of the material is spit out in a jet.

"We're interested in studying galaxies with massive black holes," said Dr. Mark Swain, a JPL astronomer and lead author of the paper. "We found that emission in NGC 4151 was unexpectedly compact. This indicates the light we saw is likely coming from a disc of material falling into the massive black hole."

The observations were made on May 19 and 20, 2003, by a team of scientists from JPL, the California Association for Research in Astronomy, and the Michelson Science Center. Akeson, Colavita, Swain and Wizinowich are part of the team.

The Keck Interferometer is part of NASA's Origins Program, which seeks to answer the questions: Where did we come from? Are we alone? The development of the Keck Interferometer is managed by JPL for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology. The W.M. Keck Observatory is funded by Caltech, the University of California and NASA, and is managed by the California Association for Research in Astronomy, Kamuela, Hawaii.