The Mysteriously Missing Quasar Gas Clouds.
One of the most fascinating aspects of studying astronomy is its many mysteries. It takes literally millions of years to watch an astronomical phenomenon occur.It’s rare that we’re allowed to capture an event that takes place in a human lifetime. In this case, it’s a quasar – not just one – but nineteen of them.
Astronomers Nurten Filiz Ak and Niel Brandt of the Pennsylvania State University and their colleagues just released a paper in The Astrophysical Journal outlining their latest study – missing quasar gas clouds. This work describes 19 distant quasars that appear to have lost their gas clouds in a period of 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,” says Filiz Ak, a graduate student at Penn State and lead author of the paper. “But why is this happening?”
Just what’s going on here? Quasars are fueled by gas input into a supermassive black hole in the middle of a galaxy. As the gas drops into the black hole, it superheats and gives off light across the spectrum. The black hole isn’t a 98 lb. weakling, either. Its gravitational force is so tremendous that the hot gas it’s sucking in radiates brighter than the entire surrounding galaxy! However, with so much happening in such a small area, some of the gas is able to escape via the intense winds blowing out from the middle of the quasar.
“These winds blow at thousands of miles per second, far faster than any winds we see on Earth,” says 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.”
How do we know the wind is there? It’s the spectra of the quasar. We’re able to measure the amount of light given off at different wavelengths from different areas. Not only are we able to identify different elements in the cloud structure, but we’re able to determine speed thanks to the Doppler effect. This leaves wide valleys which are visible in the spectrum.
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. 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.”
However, this isn’t an easy process. The center of a quasar is a “chaotic, messy place” and the rotation of the disk, combined with the speed of the winds, adds up to a quick change artist. Past research has shown examples of quasars whose broad absorption lines have disappeared between observations – something of a mystery because no one had ever attempted a specific search for them. Research like this would require measuring spectra of hundreds of quasars over a period of several years. Enter the Sloan Digital Sky Survey (SDSS). Since 1998, 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.
Thanks to the research team’s hard work, a sample of 582 BAL quasars was gathered – each one with a history of observations spanning eight years. This sample ended up being 20 times larger than anything previously attempted. The astronomers then began searching for changes and, low and behold, 19 of the surveyed quasars had evidence of disappearing absorption lines. Just what is happening?
According to the news release, “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,” says Filiz Ak.
Even though these quasar clouds are out of our range of sight for more than a century at a time, about three percent of them would seem to have disappearing gas clouds which occur over a three year period.
“Since the universe is 14 billion years old, we’re used to astronomical phenomena lasting a very long time,” says Pat Hall of York University in Toronto, another team member. “It’s fascinating to discover something that changes within a human lifetime.”
Even with these exciting new findings, there are still mysteries that will need explained and astronomers will be working on new models of quasar winds. Hopefully they will be able to shed some light on the 100-year timescale.
For now, the team will continue to analyze their quasar samples and pass on the results to the theorists.This means even more results are expected in the near future. “This 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.”
Original Story Source: SDSSIII News Release. Reported by Tammy Plotner for “Dave Reneke’s World of Space and Astronomy News”