'Missing' quasar gas clouds blown away by strong winds
A worldwide team of astronomers has finally solved the case of the missing quasar gas clouds.
The team led by astronomers Nurten Filiz Ak and Niel Brandt of the Pennsylvania State University l, described 19 distant quasars in which giant clouds of gas seemed to disappear 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 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, not all the gas is 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 professor 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," he noted.
Since 1998, Enter the Sloan Digital Sky Survey (SDSS) has been regularly measuring spectra of quasars. Over the past three years, as part of SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS), the survey has been specifically seeking out repeated spectra of BAL quasars through a program proposed by Brandt and 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 had been previously assembled. The team then began to search for changes, and were quickly rewarded. In 19 of the quasars, the broad absorption lines had disappeared.
What's going on here? There are several possible explanations, but the simplest is that, in these quasars, gas clouds that we had seen previously are literally "gone with the wind" -- the rotation of the quasar's disk and wind have carried the clouds out of the line-of-sight between us and the quasar.
And because the sample of quasars is so large, and had been gathered in such a systematic manner, the team can go beyond simply identifying disappearing gas clouds.
"We can quantify this phenomenon," said Filiz Ak.
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.
The team announced their results in a paper published in latest issue of The Astrophysical Journal.