Like an archeologist of the universe, York physics & astronomy Professor Patrick Hall studies quasars already dead for billions of years by the time their light reaches the lens of a telescope here on Earth. Hall’s quest is to uncover the nature of quasar winds and what impact they may have on galaxies, such as the Milky Way, in the future.
These quasar winds can be quite influential in the creation of galaxies. They can turn on or shut down star formation. Between two and four billion years in the future, scientists expect the Milky Way and the Andromeda spiral galaxies to collide and become one elliptical galaxy. If this occurs, the two black holes at the centre of each will likely merge and the resulting quasar winds will shut down any future star formation in the new galaxy.
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Right: An artist's conception of dust in a quasar's wind. Drawing courtesy of NASA's Jet Propulsion Laboratory at the California Institute of Technology.
“We think that this happened a lot more when the universe was younger,” says Hall.
Little is really known about quasars, never mind their winds. “We’ve only started looking at quasars in the last 50 years,” says Hall. “Most of them are quite far away and rather faint. The first discovery of a quasar was in about 1963.”
What is known is that quasars are glowing, flat discs of swirling, spiralling and colliding gas the size of the Earth’s solar system that surround a black hole, which has a mass of millions or billions times the sun's. They’re created, says Hall, when these black holes at the centre of every large galaxy draw in the surrounding gas clouds, which collide and flatten out like a giant disc. The closer the gas spirals to the mouth of the black hole, the faster it travels and the more heat it creates, which generates light and creates a quasar.
The question Hall wants answered is: what are the characteristics of the winds that can be generalized from one quasar to the next and how will that help determine how each quasar will develop and act over time?
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Left: Patrick Hall
Quasar winds can reach speeds of several thousands of kilometres per second, whipping gas away from the black holes at the centre of galaxies. Studying these winds is similar to digging up an ancient ruin. The light from one of the quasars which Hall is studying, with the help of his 2009 Natural Sciences & Engineering Research Council of Canada Undergraduate summer research assistant Konstantin Anosov, took some seven billion years to reach Earth. A lot can happen in that time – such as the formation of Earth, which is four-and-a-half-billion years old. The quasar in question is no longer in existence. A new one may have formed around the same black hole long ago – maybe several. Anyone wanting to study the newest quasar will have to wait a few billion years.
When talking quasars, the winds are matter which flies off from the disc around the quasar. The wind can be vertical or horizontal, compact or loose. Think of a blender, says Hall. “Not all the gas spirals into the black hole; like liquid in a blender, most of it spins around, but some is flung out at high velocity.”
Right: An illustration of wind from an accretion disc around a black hole at the centre of a quasar. Illustration courtesy of NASA/CXC/M.Weiss.
But seeing these winds can be tricky. Some of the gas in the wind absorbs light and can be seen, but some has been completely ionized and doesn’t absorb much light, becoming effectively invisible. The angle of viewing dictates how much or how little of the winds are visible.
One of the quasars Hall has been studying was first discovered in 1995. In 2007, the Sloan Digital Sky Survey, of which Hall is a part, looked at that quasar again and noticed the wind showed a decrease in absorption. In 2009, Hall and Anosov booked time on a telescope in Texas to have another look. This time they noticed the quasar was coming out from behind clumps of gas, and the absorption had decreased even further. It is almost possible now to see the entire light source. “So we can now get an estimate of how fast the gas clumps in the wind are moving,” says Hall. “It is an interesting enough observation…from very heavy absorption to almost no absorption in such a short time.” To see this much change in the span of 14 years is quite something.
“We know how 'big' the quasar is, so by observing how long it takes for the gas clumps to move out from in front of the quasar, we learn how fast the wind is moving across our line of sight,” he says. “I am currently working on combining that information with knowledge of how fast the wind is moving towards us to figure out how close to the black hole the wind originated.”
This leads Hall to ask what can be learned from this quasar and from watching some of the 100,000 other quasars the Sloan survey has found. “I’m trying to get a bigger picture of how the winds work and to come up with computer models to show how they work in general for quasars.” He hopes his research will provide a useful framework to use when studying quasars, providing a baseline to work from.
In the fall, Hall will be on sabbatical researching these winds to see what he can discover about quasars that were created billions of years ago.
By Sandra McLean, YFile writer