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SYFY WIRE Bad Astronomy

How Big Can a Black Hole Get?

By Phil Plait

Black holes are everywhere in the Universe. A typical galaxy might have millions of stellar-mass black holes, ones created when a massive star explodes (this is usually the kind you think of when you think “black hole”).

But some black holes are true monsters, with millions or even billions of times the Sun’s mass. We think that every large galaxy has one of these supermassive black holes in its core. Study after study has shown that these black holes have a symbiotic relationship with their host galaxies, too, growing along with them as each formed billions of years ago.

We know galaxies can get pretty big. It has to make you wonder: How big can these black holes get?

It’s a good question, and surprisingly hard to answer. Although there are a number of ways to “weigh” a black hole (really, measure its mass), but given the sheer number of galaxies out there these methods can be hard to implement on a large scale (I almost wrote “mass produced basis” but figured that’s pushing the pun too much). Despite that, we've found a few really big ones, and that can help us figure out what the upper limit to their size might be.

Now mind you, theoretically there isn’t an upper limit to them. You could, if you had godlike powers, collect every single bit of matter in the Universe, cram it into one spot, and have a truly and literally cosmic black hole with a sextillion times the Sun’s mass. More or less.

But realistically, that’s not possible. Matter is distributed throughout the Universe in the form of stars, gas, dark matter, and so on. The biggest black holes that actually exist today would probably need to be born big, and then grow over time. So the more practical question is, what’s the biggest black hole practically possible right now?

Some astronomers looked into this, and found what may be the answer: The most massive black holes likely to exist in the Universe today are about 10 billion times the mass of the Sun.

That’s still pretty dang massive. The supermassive black hole in the center of the Milky Way, our home galaxy, is around 4 million solar masses, so these monsters (called ultramassive black holes, or UMBHs) could be more than 2,000 times more massive!

How can such beasts exist?

To answer that, the astronomers looked at how black holes form in the first place, and how they could grow. In the early Universe, when it was less than a billion or so years old, things were different. Galaxies were in their earliest stages of formation, and the Universe was mostly gas and dark matter. The dark matter was distributed along huge filaments, and its gravity pulled normal matter toward it. These were the early structures, the scaffolding, upon which galaxies would form.

… and big black holes, too. Under these conditions, there are two ways supermassive black holes could’ve formed. The first is that the gas collected into huge stars, far larger than can exist today, with more than 100 or possibly even 1,000 times the mass of the Sun. These stars can’t exist today; the presence of heavy elements makes the stars too hot, and anything more than 100 or so times the Sun’s mass gets so energetic it would tear itself apart. But in the early Universe those heavy elements weren’t created yet (they form inside massive stars, as it happens), and all that was around was hydrogen, helium, and a smidgen of lithium. These primordial stars would’ve lived very short lives, blown up, and then formed huge black holes when their cores collapsed. Over time, these first black holes would draw in matter around then, growing huge.

The second way huge early black holes could form was using dark matter as a funnel. Those huge filaments of dark matter would channel normal matter down into tight knots of material. The flow could’ve been so fast that there simply wasn’t time to form a star first; instead the matter collapsed directly into a black hole. In this sense, the black holes were formed from “seeds”; a black hole was born and then grew rapidly.

Both scenarios—stars and seeds—are physically possible, and can produce a black hole with a billion times the Sun’s mass in a billion years after the Big Bang.

But that’s just the start. To get as big as we see them today, they have to grow. They can do that by simply eating more material (the dark matter that fed them initially can help there). But remember, not long after they were born these black holes had galaxies growing around them. Given longer timescales, galaxies can collide and merge. When they do, the black holes in their centers can merge as well. In clusters of galaxies, there can be thousands of galaxies all bound together by their gravity. Given reasonable growth rates, the theoretical models predict black holes can grow to 10 billion times the mass of the Sun over the age of the Universe.

And that’s how we could get ultramassive black holes today. Still, that’s theory. What about observations?

To check these numbers, the astronomers in the new study compared what they expect to see today versus what’s actually observed in galaxies. How do you measure a black hole? Indirectly: As black holes feed on matter, the material forms a swirling disk around it. This gets very hot, and glows. If the temperature reaches millions of degrees (which, terrifyingly, it can and does) it will emit X-rays, and those can be seen out to huge distances.

We see lots of these “active galaxies” in the early Universe, and they were pouring out X-rays, meaning their central black holes were feeding voraciously. During that time they were very efficient at gaining mass, and could grow huge. But how huge? And how many huge ones do we expect to see today?

There have been surveys of black holes in galaxies taken, and, for example, one of them found a handful of UMBHs out of the thousands of galaxies they looked at. It turns out that’s a problem: Assuming that brighter galaxies means more big black holes, the astronomers doing the new study predicted there should be thousands.

Obviously, something was wrong. They think that their assumption that you can simply extrapolate brightness to black hole growth doesn’t work on the high end. One explanation is that we know black holes are sloppy eaters. The tremendous amount of light the disk emits can blow a vast wind of particles outward, and that can choke off the flow of matter inward. If black holes try to eat too fast, their food dries up!

When they accounted for that, the astronomers found their numbers aligning better with observations. Interestingly, those observations indicate that the most massive black holes we see today really are about 10 billion solar masses, about what the models predicted.

How many of them do we see? Running the numbers, they predict there should be one UMBH in a volume of space encompassing 3x1026 cubic light years: a cube about 700 million light-years across one side.

That’s a ridiculously staggeringly huge volume of space, which means there aren’t too many of these monsters. There are roughly a million galaxies within this distance of us, and odds are only one has a UMBH in it.

Still, we should see a few of these monsters if we probe the Universe more deeply. That work is being done now, looking at the biggest and brightest galaxies we can, and no doubt will continue for a long time. Investigating the biggest black holes in the cosmos is enduring work. There are a lot of them to find, and they’re probably really far away.

And it’s important work, too. If the Universe limits the sizes of things, then there’s a reason for it. Maybe there wasn’t time for black holes to grow any bigger. Maybe their overall growth rate is limited. Maybe bigger ones exist and they’re so rare we haven’t seen them yet.

All of these facts tell us about the conditions in the early Universe, and how it’s changed since then to today. And all of that plays its role in the bigger picture of science itself: trying to understand how we got here, and why the Universe is the way it is. I find it positively enthralling that we can ponder and attempt to solve such problems.

And being able to think of the biggest black holes in the Universe as puzzle pieces in this bigger picture is pretty cool, too.

Oh, one final note: These UMBHs are the size they are now because the Universe is only about 14 billion years old. But time goes on, and over the next few trillion, quadrillion, and octillion years—even more—eventually these black holes will consume most everything in their galaxies, and grow huger still. They’ll outlast the stars, and possibly even matter itself! In a future so distant the numbers are almost meaningless, they will be the only large objects in the entire cosmos … and even they will eventually die. I talk about this in Crash Course Astronomy: Deep Time. If there’s a life lesson in there, feel free to ponder it.

Tip o’ the accretion disk to Randall Munroe, who mentioned this study in a What if? comic about, of all things, fireflies.

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