Case of Missing Quasar Gas Clouds Now Solved

An artist's impression of a quasar like one of the nineteen found by the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), led by Penn State astronomers. The black dot in the center represents the supermassive black hole at the center of the quasar. The red-and-yellow spiral surrounding it shows the accretion disk of hot gas falling into the black hole. Some of this gas is ejected as the quasar's wind, which is shown in light blue. The size of the accretion disk shown is comparable to the size of our Solar System. The inset at the top right shows two SDSS spectra for the same quasar (named "SDSS J093620.52+004649.2"). The upper spectrum (blue) was taken in 2002, while the lower spectrum (red) was taken in 2011. The deep, wide valley in the 2002 spectrum is a "broad absorption line" -- a feature that had disappeared from its spectrum by 2011. Credit: Chandra X-Ray Center, Nahks Tr'Ehnl, and Nurten Filiz Ak

The case of the missing quasar gas clouds has been solved by a worldwide research team led by Penn State University astronomers Nurten Filiz Ak and Niel Brandt. The discovery is being announced today in a paper published in The Astrophysical Journal, which describes 19 distant quasars whose giant clouds of gas seem to have disappeared in just a few years.

"We know that many quasars have structures of fast-moving gas caught up in 'quasar winds,' and now we know that those structures can regularly disappear from view," said Filiz Ak, a graduate student in the Department of Astronomy and Astrophysics at Penn State and lead author of the paper. "But why is this happening?"

Quasars are powered by gas falling into supermassive black holes at the centers of galaxies. As the gas falls into the black hole, it heats up and gives off light. The gravitational force from the black hole is so strong, and is pulling so much gas, that the hot gas glows brighter than the entire surrounding galaxy. But with so much going on in such a small space, some of the gas is not able to find its way into the black hole. Much of it instead escapes, carried along by strong winds blowing out from the center of the quasar.

An SDSS image of the quasar SDSS J093620.52+004649.2, one of the 19 quasars with disappearing BAL troughs. The constellation map on the bottom left shows the quasar's position in the constellation Hydra. Three successive views zoom in closer and closer to the quasar. Credit: Jordan Raddick (Johns Hopkins University) and the SDSS-III collaboration. Hydra constellation chart produced by the International Astronomical Union and Sky and Telescope magazine (Roger Sinnott, Rick Feinberg, and Alan MacRobert; http://iau.org/public/constellations/)

"These winds blow at thousands of miles per second, far faster than any winds we see on Earth," said Niel Brandt, a Distinguished Professor of Astronomy and Astrophysics at Penn State and Filiz Ak's Ph.D. advisor. "The winds are important because we know that they play an important role in regulating the quasar's central black hole, as well as star formation in the surrounding galaxy."

Many quasars show evidence of these winds in their spectra -- measurements of the amount of light that the quasar gives off at different wavelengths. Just outside the center of the quasar are clouds of hot gas flowing away from the central black hole. As light from deeper in the quasar passes through these clouds on its way to Earth, some of the light gets absorbed at particular wavelengths corresponding to the elements in the clouds.

As gas clouds are accelerated to high speeds by the quasar, the Doppler effect spreads the absorption over a broad range of wavelengths, leading to a wide valley visible in the spectrum. The width of this "broad absorption line (BAL)" measures the speed of the quasar's wind. Quasars whose spectra show such broad absorption lines are known as "BAL quasars."

But the hearts of quasars are chaotic, messy places. Quasar winds blow at thousands of miles per second, and the disk around the central black hole is rotating at speeds that approach the speed of light. All this action adds up to an environment that can change quickly.

Previous studies had found a few examples of quasars whose broad absorption lines seemed to have disappeared between one observation and the next. But these quasars had been found one at a time, and largely by chance -- no one had ever done a systematic search for them until 1998, when the Sloan Digital Sky Survey (SDSS) undertook the challenge, in 1998, of regularly measuring the spectra of hundreds of quasars during an effort spanning several years.

Over the past three years, as part of SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS), the researchers specifically have been seeking out repeated spectra of BAL quasars through a program proposed by Brandt and his colleagues.

Their persistence paid off -- the research team gathered a sample of 582 BAL quasars, each of which had repeat observations over a period of between one and nine years -- a sample about 20 times larger than any that previously had been assembled. The team then began to search for changes, and found that, in 19 of the quasars, the broad absorption lines had disappeared.

There are several possible explanations for the disappearance of the gas clouds, but the simplest is that, in these quasars, gas clouds that previously had been detected are now "gone with the wind" -- blown out of the line-of-sight between us and the quasar by the rotation of the quasar's disk and its wind. Because the sample of quasars is so large, and had been gathered in such a systematic manner, the team is able to go beyond simply identifying disappearing gas clouds. "We can quantify this phenomenon," Ak said.

Finding nineteen such quasars out of 582 total indicates that about three percent of quasars show disappearing gas clouds over a three-year span, which in turn suggests that a typical quasar cloud spends about a century along our line of sight. "It is fascinating to be able to document these relatively rapid changes that actually occurred billions of years ago, at a time before the Sun was formed," remarked team member Donald Schneider, distinguished Professor of Astronomy and Astrophysics at Penn State and the SDSS-III Survey Coordinator.

Now, as other astronomers come up with models of quasar winds, their models will need to explain this 100-year timescale. As theorists begin to consider the results, and the team continues to analyze its sample of quasars, more results are expected soon. "This research is really exciting for me," Filiz Ak says. "I'm sitting at my desk, discovering the nature of the most powerful winds in the Universe."

CONTACTS AT PENN STATE:

• Nurten Filiz Ak, Penn State University, nfilizak@astro.psu.edu, +1 814 880 7569
• Niel Brandt, Penn State University, niel@astro.psu.edu, +1 814 777 4042
• Donald Schneider, Penn State University, dschneider@psu.edu, +1 814 863-9554
• Barbara Kennedy, Penn State University Public Information Officer, science@psu.edu, +1 814 863-4682

CONTACTS AT SDSS-III:

• Michael Wood-Vasey, SDSS-III Spokesperson, University of Pittsburgh, wmwv@pitt.edu, +1 412 624 2751
• Jordan Raddick, SDSS-III Public Information Officer, raddick@jhu.edu, +1 410 516 8889

ADDITIONAL RESOURCES

Citation for the published paper, which is available free of charge on the arXiv preprint server at: http://arxiv.org/abs/1208.0836: N. Filiz Ak, W. N. Brandt, P. B. Hall, D. P. Schneider, S. F. Anderson, R. R. Gibson, B. F. Lundgren, A. D. Myers, P. Petitjean, Nicholas P. Ross, Yue Shen, D. G. York, D. Bizyaev, J. Brinkmann, E. Malanushenko, D. J. Oravetz, K. Pan, A. E. Simmons, B. A. Weaver. 2012, Broad Absorption Line Disappearance on Multi-Year Timescales in a Large Quasar Sample, The Astrophysical Journal, 757(2), 114, doi:10.1088/0004-637X/757/2/114.

ACKNOWLEDGEMENT

This material is based upon work supported by the National Science Foundation under Grant No. AST-1108604, "Large-scale investigations of quasar winds with the Sloan Digital Sky Survey-III." Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

ABOUT SDSS-III

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Penn State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.