A new study using archived astronomical observations across the electromagnetic spectrum has come to a rather startling and somewhat troubling conclusion: Supermassive black holes at the center of many galaxies may have suppressed star formation soon after those galaxies formed, effectively killing them.
Very generally, when we look around us in the near Universe today, we see two types of galaxies: those that still make stars, like the Milky Way, and those that don’t. Many of these galaxies are elliptical: cotton ball-like galaxies that have lots of stars but virtually no gas and dust, the raw fuel for making stars. And in fact, the stars in these galaxies are very old, indicating that the galaxies created stars very early on but then shut down for some reason.
Massive stars are bright and blue and don’t live long, so star-forming galaxies tend to be bluer. Galaxies with older stars look redder, so we call them “red and dead” — an anthropomorphic, but not terrible bias that star-producing galaxies are still “alive.”
But why? What killed star formation in these red, dead galaxies?
We have suspected for a while that it could be, among other things, black holes. All major galaxies have supermassive black holes at their centers, with millions or even billions of times the mass of the Sun. We don’t know exactly how huge they get, which is one of the biggest puzzles in cosmology right now. But we do know that they must have fed like raging gluttons, gobbling up matter at a ferocious rate.
When this material falls towards the galactic center, it accumulates around the black hole and becomes terribly hot. This creates a powerful wind of subatomic particles that can fly away from the black hole at phenomenal speed, with enough outward momentum to blow away the material feeding the black hole. If powerful enough, it can actually expel gas from the galaxy entirely, stopping star formation.
Is that why some galaxies die early?
To find out, astronomers looked at observations of the distant Universe from various telescopes, looking at visible light, infrared, radio and X-rays. [link to paper]. They divided the galaxies into two groups: those with active star formation and those that were quiescent. Making stars produces a lot of ultraviolet light, which is red-shifted towards the visible and infrared for galaxies very far away, so looking for galaxies bright at these wavelengths is a good way to tell them apart from galaxies that were possibly already dying at the time and producing no stars.
The active feeding of black holes produces a lot of X-rays and radio waves, so they then turned to these observations. But they ran into a problem: Quiet galaxies don’t produce much of this kind of light and don’t show up in surveys.
So they did something smart: they knew the positions of these galaxies from the other observations, so they went to the radio surveys and X-rays and looked at how bright this place was for all these galaxies. They then added up all of these observations to create an “average” galaxy, and used that to compare the two groups.
How it works? An individual galaxy does not produce enough X-rays to statistically distinguish it from the background noise, which is just random X-rays hitting the detector. But if you add a lot, on average, galactic X-rays will accumulate, unlike background X-rays. It’s a bit like tossing a coin several times and getting more heads than tails; is it statistically significant? Probably not; you have to flip it dozens, hundreds of times for the face-to-face signal to grow on the random noise of the individual flips.
When they did this with the x-ray and radio observations, they found that these distant galaxies were indeed emitting x-rays and radio waves, and more than you would expect just from the usual processes in this galaxy – many objects emit these types of light, which can be explained. This means that these galaxies have matter-engulfing black holes, which we call active galaxies.
The conclusion? Active galaxies with growing black holes were more likely to produce quiescent galaxies than those with non-feeding black holes. In other words, it appears that, on average, active black holes stifle star formation in their host galaxies, a process astronomers call extinction.
Black holes kill their host galaxies.
Again, this is the case on average. They had to add signals from many galaxies to reach this conclusion. And while they’re most likely right – that’s how I’d bet, certainly – the average washes out the detail. So this is a general conclusion that is good for classes of galaxies, but may not be valid for individuals. Some active galaxies may still have produced stars, and some with silent black holes may still be dead. The only way to overcome this is to conduct deeper surveys or look deeper into individual galaxies to better understand the vagaries of each.
This will certainly give astronomers food for thought. Supermassive black holes in galactic centers definitely affect their host galaxies as a whole; we’ve known that for decades. And we suspected that the winds from black holes could put them out, but now there’s good evidence for that. This new work could open new avenues to study this effect and try to understand why some galaxies thrive today, like our Milky Way, and others less fortunate.