When a galaxy is just a baby, forming out of a ginormous gas cloud a million light years across, two things happen. One is that the cloud collapses into stars. The other is that at the core of the galaxy a black hole forms and grows. It's unclear if it forms from the core of a supermassive exploding star (or from several that merge), or from some other mechanism. But every big galaxy has one at its heart, and it's big, and the mass of the black hole in many ways is related to the size of the galaxy itself. So we know that the formation of the galaxy itself and the black hole at its core are somehow linked.
As the black hole grows, it starts to emit a very powerful wind. This is not from the black hole itself, but from a disk of material -- called the accretion disk -- that forms around the black hole. The disk is made of matter swirling around the hole, eventually falling down the drain. This gas gets extremely hot, and may also have magnetic fields, and these combine to blast out a gale of subatomic particles.
Moreover, at some point that wind would reach out into the galaxy and blow away any incoming gas that might otherwise reach the black hole. It also blows away gas farther out that would otherwise form stars. Pretty much at that point the galaxy stops forming stars near its core, and the black hole stops growing. There's still plenty of gas in the galaxy, but it's far out from the center, out of reach of the black hole.
Or so it was thought. However, a new study out of Yale says that may not be the case. Astronomers studied young galaxies, looking at when they form stars, and also looking at when the black holes in their centers were emitting their strongest winds. What they found is that star formation in these galaxies shuts down before the black holes hit their stride, sometimes more than 100 million years before.
In other words, the situation is a little more complicated than we thought. Isn't it always?
So now the timeline looks like this: stars start to form in the nascent galaxy. The black hole in the center forms, and accumulates mass as junk falls in. The rate grows, and a wind is set up. Stars stop forming in the core of the galaxy for reason or reasons unknown, and then many millions of years later the black hole reaches its peak wind blowing capability. Sometime after that, the wind blows out the remaining gas, and we're all done. We've got galaxy.
So what's that mysterious process that kills off star birth? Well, it's mysterious: We don't know. Maybe there is some process we haven't thought of at all, a third thing happening in galactic cores that throttles back the star formation rate. Maybe the black hole wind changes with time, and its ability to do that quenching isn't linked with strength as we thought, but some other factor. Or maybe it just takes a lot less wind blowing to stop making stars. From reading the paper, it seems to me that that last one is the most likely. But it's still hard to say.
Studying young galaxies is itself a young science. We've only had the technology to do this recently; these galaxies are far away and therefore small and faint. The astronomers who did this study used Swift, a satellite that launched in 2004, to give them a good sample of galaxies (Swift has telescopes on board that can see through gas and dust and detect high-energy light coming from the accretion disks) and compare that to data from other telescopes that have only been around a few years themselves.
In other words, this type of work was literally impossible just a few years ago. But now we can do it, and learn about how entire galaxies form. This study is another step in our understanding of this enormous, complex, and colossal process.