Galactic Appetite

A team of researchers led by York University Associate Professor Patrick Hall has discovered some strange behaviour occurring in distant galaxies. Specifically, they noted several instances of gas that appears to be falling into black holes at a high velocity.

“Matter falling into black holes may not sound surprising,” says Hall, “but what we found was quite mysterious and was not predicted by current theories.”

Astronomers have known for more than a decade that every large galaxy has a supermassive black hole at its centre, with a mass from millions to billions of times larger than that of the sun. Matter around these black holes forms a disc bigger than the Earth’s orbit around the sun and hotter than the surface of the sun. These discs of hot gas, called quasars, generate enough light to be seen across the observable universe.

Using data from two components of a large survey of the night sky, known as the Sloan Digital Sky Survey, Hall and his colleagues found a new type of quasar in which gas seems to be heading away from Earth, and possibly towards the quasar’s black hole.

“The gas in the disc must eventually fall into the black hole to power the quasar, but it’s difficult to confirm that gas is actually inflowing through the disc,” says Hall. “What is often seen instead is gas blown away from the black hole by the heat and light of the quasar, heading toward us at velocities up to 20 per cent of the speed of light. These winds can affect the galaxy surrounding the quasar, and are an active topic of current research in astronomy, including my own.”

Study co-author Niel Brandt, Distinguished Research Professor at Penn State University, explains further: “Just like you can use the Doppler shift for sound to tell if an airplane is moving away from you or toward you, we use the Doppler shift for light to tell whether the gas in quasars is moving away from us or toward us. In these objects, some gas is doing both, though most of it is moving away from us.”

Quasars like these were not predicted by models of quasar winds, and astronomers weren’t on the lookout for them. “No one realized what these objects were until one day, while looking for something else, I spotted one which could only be explained as having gas around the quasar moving away from us at very high velocity,” says Hall.

Such infalling gas is believed to occur in only about one out of 10,000 quasars, and only 17 cases are known. This discovery is detailed in a peer-reviewed paper in the journal Monthly Notices of the Royal Astronomical Society, published by Oxford University Press.

Given the strong gravity of black holes, and the fact that there is nothing else unusual about these particular quasars, why isn’t gas falling into more of them?

“It could be that the gas moving away from us is not [actually] falling into the black hole but is orbiting around it, just above the disc of hot gas, and is very gradually being pushed away from the black hole,” explains Hall. “A wind like that will show gas moving both toward us and away from us.” 

Due to this discovery, models of quasars and their winds need to be revised to account for these rare cases, he says. To help understand what revisions are required, Hall, his students and his colleagues are continuing to observe these quasars further, using the Canadian and American access to the Gemini North telescope in Hawaii. ■