Astronomers Use Big New MMT in Black Hole Discovery

Feb. 22, 2001

Working independently, two teams of astronomers have used the new 6.5-meter telescope at the MMT Observatory on Mount Hopkins, Ariz., to discover a massive black hole -- the first ever found in the galactic halo, thousands of light years above the Milky Way galactic plane.

"We knew after our first half-night observing run that this object had to be a black hole at least five times as massive as our sun," said R. Mark Wagner of the Large Binocular Telescope Observatory and the Steward Observatory at the University of Arizona.

Wagner, Craig B. Foltz, director of the UA/Smithsonian Institution MMT Observatory (MMTO), and Sumner Starrfield of Arizona State University collaborated on one team that used the newly upsized telescope to study the system starting last November.

Jeffrey E. McClintock of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., led another team who also used the MMT to observe the explosively X-ray bright system at about the same time.

"The fact that two teams were poised and ready to observe the object as soon as it emerged from the sun's glare is an indication of how potentially interesting this object is. We were sure that astronomers all over the world were getting ready to observe it," Foltz said.

But, neither team was aware of the other's plans.

"Both our groups used the same telescope, the same spectograph, and the same grating. We analyzed our observations separately but came up with the same results, as we hoped would happen," Wagner said. Both teams are reporting their results in an issue of Astrophysical Journal Letters.

"It is an amicable competition," Foltz said. "The groups are in frequent contact and we are all friends."

"In fact, Craig and I have collaborated on similar projects at the MMTO in the past," McClintock said.

The black hole is at least six times as massive, possibly closer to eight times as massive, as our sun. It lies approximately 6,000 light years away, high in the galactic halo, 62 degrees above the plane of our Milky Way. The black hole betrays its presence by the effect it has on the motion of a normal star, smaller than our sun, in orbit around it. The star is slowly transferring matter to the black hole.

"The period of the orbit is short -- only 4.1 hours -- and this makes the orbit decay 'quickly,'" in terms of cosmological time, McClintock said. "We expect that in about 2 or 3 billion years, the normal star will reach the black hole and be consumed."

"We are intrigued to find such an object in the galactic halo," Wagner said. "Because this is so far above the galactic plane, there is almost no interstellar medium between us and the object, so we can study it in detail like no other object in its class," he said.

Because the system is faint -- more than 160,000 times fainter than can be detected with the unaided human eye -- it took a precision giant telescope to get optical spectra (separated colors of light) in only a few hours time. And a few hours each night was all each team had to view their target, which in November rose in the sky well after midnight.

Wagner and Foltz are elated by technological as well as scientific success.

The giant MMT, formerly a six-mirrored instrument equal to a 4.5-m telescope, is a joint facility of the University of Arizona and the Smithsonian Institution. It now features a 6.5-meter, stiff, lightweight borosilicate "honeycomb" mirror spin-cast and polished at the UA Mirror Lab. The new telescope was dedicated in May 2000 and scientific observations with it began in earnest only last fall.

"It is really exciting to have such an exciting discovery so early in the telescope's new life," Foltz said.

"As hoped, we learned that the new MMT excels in applications where you need a lot of light, " Wagner said. "You ask a lot of your telescope to hold the target star in the spectrograph's very tight slit. We learned that the telescope is stable, that it tracks very well, and that the image quality is good enough to do very precise spectography on a very faint object. "

The alternately X-ray quiescent and explosively X-ray bright system now known to be the first high latitude black hole yet found was discovered in March 2000 by the Rossi X-ray Timing Explorer, a NASA X-ray observatory that is uniquely suited to the discovery and study of such black holes.

The system is X-ray bright because X-rays are produced by gas falling from the normal star toward the black hole, McClintock said. The gas near the black hole is heated by friction and gravity to millions of degrees, and the heat energy is radiated as X-rays.

"The system now is quiescent,"said Michael Garcia, a member of the SAO team, "which allowed us to see the normal star and thereby measure the mass of the black hole."

Ten other less massive such systems all lie in the Milky Way galactic plane. Another 20 more X-ray emitting sources in the middle of the galactic plane also could be black holes, but are too faint to optically study through the shroud of bright stars and the interstellar medium.

Link to the MMT Observatory web site, http://sculptor.as.arizona.edu/foltz/www/mmt.html