The case of the missing quasar gas clouds has been solved by a worldwide research team led by Penn State astronomers Nurten Filiz Ak and Niel Brandt. The discovery was announced Oct. 1 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.
"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 doctoral adviser. "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 19 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 said. "I'm sitting at my desk, discovering the nature of the most powerful winds in the Universe."
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