Thar Be Dragons
There is a dragon, of sorts, at the heart of every galaxy. Without exception, evidence has been found for a massive black hole at the center of every galaxy for which it’s possible to find evidence. And how big is “massive”? Pretty big. The black holes at the center of the Milky Way and Andromeda galaxies are reckoned to be from 5 to 30 million times the mass of the sun, and some make those two seem small. We should consider ourselves lucky that the thing is sitting there 5000 light-years away, starved for food and quiet for now.
Why shouldn’t it be quiet?
There’s only a couple of times when a black hole emits scads of radiation – when it gets formed (presumably surrounded by lots of raw star stuff), and when it meets up with another galactic black hole (which happens when galaxies collide, as they sometimes do). You see, black holes are very sloppy eaters and spew radiation all over the place when they feed. In a newly formed galaxy they emit enough to be seen across the known universe – and enough, it seems, to sterilize the surrounding galaxy. The dragon devours all in the village. So maybe it’s a good thing that the black hole is most active when the village is least populated, or not populated at all.
But maybe not. Maybe galactic black holes find a way to feed at other times when there are lots of towns folk with a cause to worry.
The relatively quiet black hole at the center of our Milky Way galaxy could one day reignite, spewing forth so much radiation that the sky would never darken. That grim scenario has become more likely based on a new survey of galaxies hosting active black holes at their centers.
Well that could ruin your day.
Astrophysicist Paul Westoby and colleagues from Liverpool John Moores University in the U.K. used data from the Sloan Digital Sky Survey to survey spectral lines from 360,000 relatively nearby galaxies. As Westoby reported last week at the National Astronomy Meeting in Belfast, some 70,000 of those galaxies–nearly 20%–definitely showed active nuclei, and 65,000 more were listed as “probable AGN,” Westoby says. It’s not understood what is causing the black holes to become newly active, because in most cases there is no evidence of collisions or mergers.
AGN is “Active Galactic Nuclei”.
Don’t panic just yet. It’s quite possible that Westoby & Co. have used a definition of “active” that make the statistics seem a lot worse than they really are. But it’s an interesting study nonetheless.