Ancient quasars in distant galaxies caught switching on suddenly
Most cosmic events happen on huge timescales. Not so for quasars – the bright centres of galaxies that are powered by supermassive black holes gobbling down gas and dust. We have just seen them ignite in a matter of years.
Astronomers expect quasars to use up their fuel and settle down into quiet galaxies – a process that should take hundreds of thousands of years. So last year, when a dozen quasars were spotted shutting down in just hundreds of days, it was a shock.
Chelsea MacLeod of the University of Edinburgh, UK, and her colleagues wondered if these objects might turn on again. The team compared images of galaxies from the Sloan Digital Sky Survey (SDSS) with images of the same objects from the Pan-STARRS survey, taken 10 years later. After flagging 1000 objects that varied in brightness from one survey to the next, the team pinpointed five galaxies that appeared to shape-shift into quasars.
What’s more, one quasar appeared to turn on several years after turning off. But it’s not clear if the other four are turning on for the first time, or if they are also flickering.
The real surprise is the timescale. Previously, astronomers thought it should take thousands to millions of years to funnel enough gas on to a supermassive black hole for it to spawn a quasar. It definitely shouldn’t take less than 10.
“This is a bit of an embarrassing moment for black hole and quasar scientists,” says Eric Morganson of Harvard University. “The conventional wisdom was found to be dramatically wrong.”
Now the big question is how these beasts ignite in the first place. The most common explanation has two galaxies colliding, sending gas and dust swirling into the merged supermassive black hole. But the SDSS galaxy images show no sign of recent collisions, so something else must be acting as a funnel for the gas.
Some kind of instability in the galaxy itself, such as a central bar or spiral arms that push stars out of their circular orbits, could provide the funnel, suggests Mike Eracleous of Pennsylvania State University. But according to models this happens on much longer timescales than decades.
Future studies will hopefully shed light on such a rapid process, which in turn affects how galaxies, like the Milky Way, form and evolve. “Understanding how [quasars] work is really part of the puzzle of how we went from the big bang to galaxies to stars to planets,” Morganson says.