Their conclusions, reported Jan. 8 at the American Astronomical Society meeting in Kissimmee, Florida, clarify why quasar SDSS J1011+5442 changed so dramatically in the handful of years between observations.

“We are used to thinking of the sky as unchanging,” said 91̽»¨ astronomy professor , who is principal investigator of the SDSS’s Time-Domain Spectroscopic Survey. “The SDSS gives us a great opportunity to see that change as it happens.”

Quasars are the compact area at the center of large galaxies, usually surrounding a massive black hole. The black hole at the center of J1011+5442, for example, is some 50 million times more massive than our sun. As the black hole gobbles up superheated gas, it emits vast amounts of light and radio waves. When SDSS astronomers made their first observations of J1011+5442 in 2003, they measured the spectrum of the quasar, which let them understand the properties of the gas being swallowed by the black hole. In particular, the prominent “hydrogen-alpha” line in the spectrum revealed how much gas was falling into the central black hole.

The SDSS measured another spectrum for this quasar in early 2015, and noticed a huge decrease between 2003 and 2015. The team made use of additional observations by other telescopes over those 12 years to narrow down the period of change.

“The difference was stunning and unprecedented,” said 91̽»¨astronomy graduate student , a member of the research team. “The hydrogen-alpha emission dropped by a factor of 50 in less than 12 years, and the quasar now looks like a normal galaxy.”

The change was so great that throughout the SDSS collaboration and astronomy community, the quasar became known as a “changing-look quasar.” The black hole is still there, of course, but over the past 10 years, it appears to have swallowed all the gas in its vicinity. With the gas fallen into the black hole, the SDSS team were unable to detect the spectroscopic signature of the quasar.

“This is the first time we’ve seen a quasar shut off this dramatically, this quickly,” said , a postdoctoral researcher at Pennsylvania State University.

Before Runnoe, Ruan and their colleagues could come to this conclusion, they had to rule out two other possibilities. A thick layer of dust could have passed through the host galaxy, obscuring their view of the black hole at its center. But, they concluded that there is no way that any dust cloud could have moved fast enough to cause a 50-fold drop in brightness in just two years. Another possibility is that the bright quasar in 2003 was just a temporary flare caused by the black hole ripping apart a nearby star. While this possibility has been invoked in similar cases, it cannot to explain the fact that the changing-look quasar had been shining for many years before it turned off.

The team’s conclusion is that the quasar has used up all the glowing-hot gas in its immediate vicinity, leading to a rapid drop in brightness.

“Essentially, it has run out of food, at least for the moment,” says Runnoe. “We were fortunate to catch it before and after.”

The changing-look quasar is the first major discovery reported for the Time-Domain Spectroscopic Survey, one component of SDSS’s fourth phase, which will continue for the next several years.

“We found this quasar because we went back to study thousands of quasars seen before,” said Anderson. “This discovery was only possible because the SDSS is so deep and has continued so long.”

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For more information, contact Runnoe at jcr26@psu.edu or 814-863-9343, Ruan at jruan@astro.washington.edu or 206-543-5185 and Anderson at anderson@astro.washington.edu or 206-685-2392.

Download animation:  (credit: Dana Berry/SkyWorks Digital, Inc.). This animation shows an artist’s conception of the changing-look quasar as it evolved from 2003 to 2015. The beginning of the animation shows gas falling into the central black hole, along with the first SDSS spectrum. The black hole then uses up all the surrounding gas, and it is shown with the spectrum recently obtained by the Time-Domain Spectroscopic Survey. The camera then pulls back to reveal the entire galaxy as the quasar shuts off, after which is looks like just another normal galaxy. The animation then fades into an artist’s conception of Hanny’s Voorwerp, a prior SDSS discovery that shows the record of a similar quasar shutting off.

Adapted from “The Case of the Missing Quasar,”  prepared by Jordan Raddick for the American Astronomical Society and the Sloan Digital Sky Survey.

The , of which the 91̽»¨ is part, will soon see the entire sky, and even peer into the Milky Way’s galactic center.

The sky survey, called SDSS for short, is a multi-institution group of astronomers who since 2000 have searched the skies with the 100-inch, wide-angle optical telescope at the Apache Point Observatory in New Mexico.

“We have mapped the large-scale structure of the universe, traced out previously unknown structures in the Milky Way, and made unanticipated discoveries from asteroids in our own solar system to the most distant quasars,” said Michael Blanton, an astrophysicist with New York University and director of the new phase of the survey.

Now, the group is adding a second participating telescope high in the Chilean Andes, and a program using an innovative technology — aided by 91̽»¨engineers — to observe and make detailed maps of thousands of previously unstudied nearby galaxies.

“The SDSS has observed more than half a million Milky Way stars over the past 14 years, which I call a good start,” said Jennifer Johnson, astronomer at The Ohio State University and the scientific spokesperson for the project. “However, from the Northern Hemisphere, the Earth blocks our view of a quarter of the Milky Way, and mostly obscures our view of the galactic center.”

Completing that picture will be the Irénée du Pont Telescope at Las Campanas Observatory in Chile, home to the clearest skies on the planet. The second telescope will also study stars in the nearby , giving astronomers a better understanding of the Milky Way’s immediate neighborhood.

The sky survey’s new program, dubbed , is a set of three surveys involving a collaboration of 200 astronomers across 40 institutions and four continents. The work is made possible in part by a new technique of bundling sets of fiber-optic cables into tightly-packed arrays. These collect light from across an entire galaxy, enabling detailed measurements of more than 10,000 nearby galaxies at 20 times the rate of previous surveys.

Staff and students in the , led by research scientist Nick MacDonald, are active in the design, engineering and testing of the innovative fiber-optic arrays.

The new surveys will enable astronomers to:

• Explore the compositions and motions of stars across the entire Milky Way in unprecedented detail.

• Measure the expansion of the universe during a poorly understood five-billion-year period of the universe’s history.

• Make detailed maps of the internal structure of thousands of nearby galaxies to learn how they have grown and changed over billions of years.

Another program in this new phase will follow up on objects whose light output varies with time.

“Such investigations discover and characterize a menagerie of astrophysical extremes,” said Scott Anderson, 91̽»¨Astronomy Department chair and co-lead on that survey, joined by researchers from other participating institutions and assisted by 91̽»¨graduate student John Ruan.

“These range from flaring or pulsating single stars and binary star systems in our own Milky Way to luminous quasars powered by material spiraling into supermassive black holes at the centers of distant galaxies.”

The 91̽»¨was among the first eight members of the sky survey group. The telescope, optics and buildings at the Apache Point Observatory were designed at the UW. Former 91̽»¨astronomer Bruce Margon, now with the University of California, Santa Cruz, was the survey’s first scientific director.

Margon’s words when the sky survey began are no less true today. Calling it “an encyclopedia of the sky,” he added, “the possible projects resulting from the information are never-ending.”

Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation and the participating institutions.

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This was adapted from a by the Sloan Digital Sky Survey. For more information on the UW’s contributions, contact Anderson at sfander@u.washington.edu.

• Watch “The SDSS at Night,” a video by John Parejko of Yale University and the Sloan Digital Sky Survey collaboration. Workers are seen working on and studying the heavens with the Sloan Foundation Telescope at Apache Point Observatory, Sunspot, New Mexico.