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Astronomers detect plasma at black hole

September 5, 2003

UCLA astronomers report they have detected remarkably stormy conditions in the hot plasma being pulled into the monstrous black hole residing at the center of our Milky Way galaxy, 26,000 light years away. This detection of the hot plasma is the first in an infrared wavelength, where most of the disturbed plasma's energy is emitted, and was made using the 10-meter Keck II Telescope at the W.M. Keck Observatory in Hawaii.

Plasma is a hot, ionized, gas-like matter -- a fourth state of matter, distinct from solids, liquids and gases -- believed to make up more than 99 percent of the visible universe, including the stars, galaxies and the vast majority of the solar system.

"Previous observations at radio and X-ray wavelengths suggested that the black hole is dining on a calm stream of plasma that experiences glitches only 2 percent of the time," said Andrea Ghez, professor of physics and astronomy at UCLA, who headed the research team. "Our infrared detection shows for the first time that the black hole's meal is more like the Grand Rapids, in which energetic glitches from shocked gas are occurring almost continually."

"I see this as a real breakthrough," said Mark Morris, a UCLA professor of physics and astronomy, who worked with Ghez. "It's a big leap, not just an incremental advance. The infrared is precisely where we need to look to learn what the black hole is eating. In the infrared, you see it all. The black hole's dirty laundry is hanging right there for us to see. We're peering deep down inside this tumultuous region."

"One of the big mysteries in studies of the black hole at the center of our galaxy is why the surrounding gas is emitting so little light compared to black holes at the center of other galaxies," Ghez said. "We now have a completely new and continuously open window to study the material that is falling onto the black hole at the center of the Milky Way."

The past two years, Ghez and her colleagues used adaptive optics at the Keck Observatory to get high-resolution images at wavelengths between the short near-infrared, where stars dominate, and the mid-infrared, where dust dominates.

"There's a history of false detections of this source in the infrared," Ghez said. "At short wavelengths, it's challenging because there are so many stars. In the mid-infrared, it's difficult because there is so much dust at the center of the galaxy. Our observation was successful because it was made between these two problematic regimes with an adaptive optics system. This type of observation only became possible last year."

"We are highly confident in our detection," Ghez added. "We have a bright source at exactly the right spot, right on the black hole, and with properties that are unlike the stars around it; the source emits much more strongly at long wavelengths than the stars, and the source doesn't move, while the stars move at huge velocities. What's exciting and important is not just that we detected the plasma, but that it varies dramatically in intensity from week-to-week, day-to-day, and even within a single hour. It's as if we have been watching the black hole breathing."

Black holes are collapsed stars so dense that nothing can escape their gravitational pull, not even light. Black holes cannot be seen directly, but their influence on nearby stars is visible, and provides a signature, Ghez said. The black hole, with a mass more than three million times that of our sun, is in the constellation of Sagittarius.

Since 1995, Ghez has been using the W.M. Keck Observatory's 10-meter Keck I Telescope atop Mauna Kea in Hawaii -- the world's largest optical and infrared telescope -- to study the galactic center and the movement of 200 nearby stars. She has made measurements using a technique she refined called infrared speckle interferometry, and for the last few years, has used adaptive optics, an even more sophisticated technique, which enables her to see the region more clearly.

"The Keck Observatory is one of the best facilities in the world for this research," Ghez said.

The astronomers know the location of the black hole so precisely "that it's like someone in Los Angeles who can identify where someone in Boston is standing to within the width of her hand, if you scale it out to 26,000 light years," Ghez said. The galactic center is located due south in the summer sky.

The black hole at the center of our galaxy came into existence billions of years ago, perhaps as very massive stars collapsed at the end of their life cycles and coalesced into a single, supermassive object.

For decades, the emission at the galactic center could be detected only in radio wavelengths, which do not reveal the variations in intensity. "The radio is partially opaque," Morris said. The emission was detected for the first time recently in the X-way wavelengths, but it is important to now have the detection between these two wavelength extremes, where details of the plasma can be seen. In the X-ray, activity can be seen only about 5 percent of the time, while in the infrared, it can be seen continually, Morris said.

The astronomers are learning what is causing gas to emit radiation as it approaches and enters the black hole. Ghez and her colleagues will continue to study the supermassive black hole at a variety of near infrared wavelengths.

Ghez's co-authors include Morris; UCLA physics and astronomy professor Eric Becklin, who identified the center of the Milky Way in 1968; California Institute of Technology research scientist Keith Matthews, and UCLA graduate student Shelley Wright.

University of California - Los Angeles