A few weeks ago there was a fairly cool paper on arXiv that suggested that supermassive black holes may only be able to grow so big.

This triggered several Astronomy Cast listeners to write and basically say “Huh?” We addressed this a little bit in today’s questions show (real show to follow tomorrow or Wednesday – we had problems with a corrupt audio file over the weekend and got behind). I wasn’t able to give as thoughtful a response in the show as I might have liked however, so I’m going to try and write something here.

So, first, I’d like to say there are two ways to look at this: 1) In reality, 2) in make-believe land.

Make-believe land is oh so much more fun. So, lets imagine that somehow we are able to grow a very large blackhole in isolation. Then, using imaginary technology (we are in fantasy land, afterall), we throw a star at the supermassive blackhole (SMBH) so that it’s goes on a straight, uninterrupted path toward the SMBH. So straight, so perfect, infact, that if we could watch we’d see it hit on a line connecting the star’s center of mass with the SMBH’s center of mass. Now, the SMBH will simply slurp up this perfectly thrown star. Burp. No more star and no accretion disk. Now, If you, using your super duper, impossible, imaginary technology could throw a star with dead on aim over and over every second across all the epochs of time, you could pretty much build a SMBH as big as you wanted.

Here is where I stress this pretty much can not happen.

Let’s look at the reason’s briefly. First of all, the only way to easily get a SMBH all by itself is to have three galaxies merge together and one of the SMBH’s in the merging system gets ejected via three body interactions. (Pretty much any three body interaction between objects of similar masses leads to one of the masses getting spit out. This works for stars and black holes). So now I have an isolated SMBH hurtling itself through the intergalactic medium, passing rapidly through the space between galaxies. Yes, I can get a SMBH all by itself, but now I kind of have it hurtling through space.

So let’s image it lands in a nice friendly crowd of stars. As it hurtles in  through the stellar populations, stars will fall in from all sides, spiraling in and forming a disk. At the same time, any dust and gas that may be around will also get sucked in. Eventually, a thick disk of material from this thick pocket of stuff will form a nice hot disk that give off so much light and wind that anything new trying to fall in will simply get blown away before it can get so close that it can’t escape.

And it’s not like there is a lot of stuff to eat in this imaginary star cluster we have flung our imaginary SMBH into.

The reality of the universe is in many ways far more interesting then our imaginary scenerio and galaxies self-regulate the size of supermassive black holes through complex feedback mechanisms. In the rather cool paper mentioned above, astronomers Priyamvada Natarajan and Ezequiel Treister describe how a combination of winds and star formation regulate the size to which SMBHs can grow. They estimate the upper limit is roughly 10^10 Solar masses (10,000,000,000 Solar Masses).

Here’s how it works:

First of all, most galaxies just don’t have enough stuff around them to ever grow that big, and as the universe expands and carries non-gravitationally bound systems farther and farther apart, the potential for truly giant SMBH’s dimishes.

Second, while it is possible to grow what they term Ultra-Massive Black Holes, SMBH’s approximately the size of the 10^10 Solar Mass upper limit, it is hard to grow them bigger. These objects when they do exist, are in the hearts of cD galaxies sitting in the centers of large galaxy clusters. In these systems, the gas the SMBH can consum eventually gets pushed away when the luminosity of the accreting material reaches a certain threshold. Once the gas is pushed away, it really has no reason to fall back to the center and get eaten later unless something disturbs its orbit, and if that does happen, it will get pushed away again as soon as the accrestion luminosity again gets above a certain level.

In addition to clearing out dust and gas that isn’t currently trying to flow in, but happens to be local, this accretion luminosity can also actually stop material that is in the process of falling in already (stuff that hasn’t gotten to the event horizon yet). It’s all about how the kinetic energy of the infalling material is changed.

Finally, the growth of the SMBH into an UMBH also means that any outflow from the black hole will be bigger and badder. In general, the material flowing away from SMBH’s flows out, cools, and some of it condenses into stars or flows back in as accretion. Once the outflow get’s big enough, it becomes a superwind and no recycling of material takes place – bye  bye gas.

So, in our reality, black holes have an upper limit that comes basically from the fact that things spiral in and the material spiralling in does destructive things: Blasting Light, Blasting Wind. Blame angular momentum and if you want to build a bigger blackhole, get good at aiming stars.