Which came first, the galaxy or the black hole?
Every big galaxy we see in the sky has a supermassive black hole at its heart, a dark monster that may be millions or billions of times the mass of the Sun. But did the black hole form first, or the galaxy… or did they grow together?
Astronomers think they may be on to the answer: black holes form first, or at least more quickly, and galaxies grow around them.
It’s been known for a decade or two that big galaxies have giant black holes in their cores. It was a surprise at first, but now it’s expected that they all do. The Milky Way certainly does; our black hole is 4 million times the mass of the Sun, and it sits at the geometric center of the galaxy. Every time we look carefully at big galaxies, we see evidence for such a beast.
The second surprise came when the masses of the black holes were compared to the masses of their host galaxies. No matter how big or how small the galaxy, the mass of the black hole it harbored scaled right along with it (or, more accurately, with the bulge of stars, gas, and dust at the galactic center… sortof like the downtown region of a big city). Galaxies with bigger central bulges have bigger black holes, smaller galaxy bulges means more modest black holes.
This shocked astronomers. Mind you, as scary and big as these galactic black holes are, they are still a tiny fraction of the mass of their host galaxy’s bulge, about one-tenth of a percent, in fact. And that’s just for the bulge; the black hole is only about a thousandth of a percent of the total mass of the entire galaxy. So the black hole is downright dinky compared to the galaxy. How could it possibly affect this gigantic structure around it?
This scaling issue means that somehow, the black hole and the galaxy must “know” about each other; either the black hole affected how big the galaxy got, or the galaxy itself somehow shaped the size of the black hole, or some third characteristic shaped them both. But which was it?
One way to figure out which came first, the galaxy or the black hole, is to look at very distant galaxies. When we look at nearby galaxies we see this scaling between the galaxy and its black hole. But in these cases, the galaxies are old compared to the age of the Universe; we’re looking at 10-12 billion year old galaxies in a 13.7 billion year old Universe. By now, most galaxy and black hole-forming processes have settled down and stopped.
But if we look at very distant galaxies, we see them as they were when they were much younger, only a billion or two years old. If black holes grow more quickly or more slowly than their host galaxies, then looking that far back in distance and time may make that easier to see. This is what Chris Carilli, of the National Radio Astronomy Observatory (NRAO), and an international team of astronomers did.
When they looked at distant, young galaxies, they found that the black holes were more massive relative to their host galaxies than they are today! That’s a big result. It implies very strongly that somehow, the black holes grew first, with the galaxies growing more slowly around them.
This is the first big breakthrough in the galaxy/black hole chicken-and-egg problem. It’s a key finding that will allow astronomers to pursue the myriad questions that we still have… like, do the formations of the galaxies and black holes start at the same time, but black holes grow more quickly? Does the black hole reach its adult size and stop while the galaxy is still growing? And the real killer question: what the heck process is going on that relates the final mass of the black hole to the mass of the galaxy?
We still don’t know. But we have ideas… one is that black holes are messy eaters. As matter falls onto the nascent hole, it forms a flattened disk. This channels a vast wind that blows out from the superheated material in the disk that is just above the black hole’s point of no return. This gale of subatomic particles and energy blows out into the galaxy, affecting how stars form and how material from the proto-galactic cloud falls onto the galaxy itself. Eventually, the black hole runs out of material to feed on, the wind shuts off, and the growth stops… but at the same time, that wind has blown away all the material the galaxy was using to grow and form stars.
So this process would link the growth and eventual size of the black hole to the far larger galaxy around it. The thing is, this is just an idea; we don’t know if it’s right or not! But this new work studying distant galaxies has provided a vital piece of evidence to what’s actually going on. Now astronomers can focus their attention on these galaxies and see if they can tease out more details, more clues so that they can solve one of the biggest outstanding cosmic mysteries today: how, specifically, did galaxies form?
Or, if you prefer: how did we get here? After all, almost every question in astronomy boils down to this one. And every day, we get a bit closer to the answer.