A Limit To Supermassive Black Hole Growth
It looks like the most voracious beasts of the universe, supermassive black holes, may have an upper limit to their appetites. Calculations show that beyond 50 billion solar masses, they go on a permanent hunger strike because their buffet table disappears.
Supermassive black holes are the wonders of the cosmos. They are far and away the most massive discrete objects in existence. Ancient and distant quasars were the first solid evidence of their existence when it was realized they can release a trillion time the energy of our sun in a space similar in size to our solar system. These tiny regions in Active Galactic Nuclei can outshine all the stars in its entire host galaxy.
These behemoths are just one of a trio of standard black hole classifications. Ordinary stellar mass black holes weigh-in at 33 times the mass of our sun or less. Less prevalent are the intermediate mass black holes which generally have masses below 100,000 Solar Masses (SM). Supermassive holes are anything above that extending into the millions and even billions of times the mass of our sun. But how many solar masses can they possibly contain in their mysterious singularity? Is there an upper limit?
This question was recently investigated by researchers and published in the Monthly Notices Letters of the Royal Astronomical Society. Their calculations were mostly concerned, not with the black hole proper, but the accretion disk often found around them supplying them with matter to consume. This accretion disk is what makes many black holes so easy to spot in the first place. Gas and dust and other detritus swirl around the black hole, moving faster and faster in ever tighter orbits and getting hotter and hotter. This friction releases boat-loads (or is it a shit-ton?) of energy (primarily x-rays) that can be seen clear across the observable universe. These are the Active Galactic Nuclei or Quasars that you may heard astronomers going on about.
There’s much we still don’t know about how these biggest holes got so big, but gaining mass through these accretion disks clearly plays a prominent role. It seems however that accretion disks become increasingly unstable the bigger the supermassive black hole becomes. This instability appears to reach a breaking point around 50 billion solar masses causing the disk to completely fragment and collapse into stars in their own rights.
This is a critical milestone because such black holes can never again be luminous accretors. They can never create or be detected as Active Galactic Nuclei while they suck in their disk of matter growing with each meal to become even more….supermassive.
I was a little bummed when I first read about this. Especially considering that we’ve already detected such monsters near this mass range. Sure, 50,000,000,000 suns worth of mass is impressive as hell but it’s kind of like learning that the Hulk can’t get any madder or bigger or stronger than he already has.
Ahhh, but if you’ve read carefully, I said that such a beast can never again be a luminous accretor. It can no longer dine on a glowing accretion disk and get bigger from it. There are other ways they can pig out though….Black hole mergers
It’s entirely possible then for two supermassive black holes, each at 50 billion solar masses, to merge and create a 100 billion mass monstrosity. Sure, this could never become luminous and therefore easy to find but there are other ways to detect them such as gravitational lensing.
So, that made me feel better about this and…wait a second…imagine if two 100 billion solar mass black holes merged!!!
Image Credit: Astrobites.org