How To Capture Black Holes (Script) (Patreon)

Black holes are awesome - but our universe kindly requests once again that you hold its beer. How about black holes being captured by the screaming vortex of a quasar, where they merge and grow like some monstrous version of a solar system. This insane hypothesis is getting closer to reality, according to the papers in today’s space time journal club.

In September 2015 the laser interferometer gravitational wave observatory - LIGO - detected its first gravitational wave from the merger of two black holes. That was stunning enough, but the real promise lay ahead. Every time we learn to observe the universe in a new way we discover new things. When we figured out how to see in radio waves quasars and supernova remnants lit up the sky, When we learned to see in neutrinos the core of the Sun became visible to us. But now that our vision extends to gravitational waves, what else might we discover?

The black hole mergers themselves were not so surprising. Einstein’s general relativity predicted gravitational waves and astrophysics predicted black hole mergers. When two very massive stars are in binary orbit with each other, they may end their lives to leave a pair of binary black holes. And in very dense environments like the cores of galaxies, lone black holes may find each other and form a binary pair. However the pair forms, it can’t last forever. As they circle each other, black holes whip the fabric of space into expanding ripples - gravitational waves - which saps orbital energy from the system. The black holes spiral closer and closer together. In the last instant they coalesce into a single black hole, and the powerful gravitational waves produced in the last fraction of a second are what LIGO detects - sometimes from over a billion light years away.

We expected to see black hole mergers, but there was some striking surprises. For one thing, many of the merging black holes were too massive to have been formed by the collapse of stellar cores. That is if our understanding of stellar evolution is half as good as we think it is. This led astrophysicists to think about new ways to produce black hole mergers. Here’s the most awesome possibility: what if black hole mergers actually occur in orbit around supermassive black holes, embedded deep in the whirlpools of searing gas that surround some of these monsters? Today on Space Time Journal Club we’ll be looking at a pair of 2019 papers that talk about this possibility. We have Yang et al., which predicts the properties of black holes that merge this way, and McKernan et al. which proposes a way for us to actually test this hypothesis.

The argument goes like this: we know that the center of almost every galaxy contains a supermassive black hole of millions to billions of times the mass of the Sun. Recently we’ve also learned that the galactic center likely also contains a swarm of perhaps tens of thousands of stellar-mass black holes. These are the remnants of dead stars, typically a few to a few tens times the mass of our Sun. They rained down on the galactic center over billions of years as massive stars formed and died in the surrounding galactic core. This has been a theoretical prediction for some time, but we’ve recently found evidence of the Milky Way’s black hole swarm - and yeah, we covered that in a previous episode.

OK, so, a black hole swarm surrounding a supermassive black hole sounds like a recipe for black hole collisions. Actually not so much - black holes are so compact that they never collide outright - they need to merge by first forming a binary pair and then falling together. Now binary black hole pairs surely do exist in the dense galactic center, but they may have trouble merging in such a dense environment. Regular glancing encounters with other stars or black holes can tear binary pairs apart before they can spiral together. 

But there’s a way to massively accelerate the mergers of these black holes: all you need is a little quasar. For the most part the supermassive black holes at galactic centers are, well, black. But occasionally gas from the surrounding galaxy will find its way into the galactic center and form an incandescent vortex - an accretion disk - as it plummets into the insane gravitational field of the central monster. In the case of the largest, most well-fed black holes this results in the quasar phenomenon, and their accretion disks glow bright enough to be visible from the other side of the universe. More generally, these feeding supermassive black holes are called active galactic nuclei - AGNs.

This is how supermassive black holes can grow to such enormous sizes, but what does the presence of an accretion disk mean for the swarm of stellar mass black holes? The orbits of those black holes are mostly random, so the swarm forms a spheroid a few lightyears across. The accretion disk is quite a bit smaller than the full swarm, but there should still be plenty of black holes orbiting in the region. These will punch right through the accretion disk twice every orbit. On each pass a streamer of gas is dragged out of the disk, tugged by the black hole’s gravitational field. Momentum is transferred from black hole to gas, slowing the black hole down a bit and causing its orbit to decay - much like how a satellite’s orbit decays if it’s too close to Earth’s atmosphere.

Eventually these disk-crossing black holes should be swept into the accretion disk. There they gorge on the gas of the disk and grow in mass much, much faster than they could in almost anywhere else in the galaxy. So now we have one way for these black holes to get big. But in order to be detected by LIGO, they also need to merge with each other. There are two ways this can happen: If a binary black hole pair gets captured by the disk, the surrounding gas saps their orbital energy much more quickly than by gravitational radiation alone. This means they can spiral together before being ripped apart by a glancing blow with another object.

And accretion disks also allow lone black holes to find each other. This is really cool, because the process is similar to how planets form. When a massive object is embedded in a rotating disk, it will exert a gravitational tug on the surrounding particles. Depending on the local properties of the disk, that can cause the object to either gain or lose angular momentum. If it gains angular momentum the size of its orbit increases, so it moves further out in the disk - or “migrates outward”. If it loses angular moment it migrates inwards. But the local properties of a disk will change with the distance from the center. In some regions you get inward migration and in some regions outward migration. The boundaries between these regions are called migration traps - no migration occurs there. So an embedded object will wander inwards or outwards until they find one of these traps, and they’ll remain stuck there for some time.

In infant planetary systems, a disk of gas and dust surrounds the newly-formed star - a protoplanetary disk. Lumps of coagulated ice and dust migrate to these traps, where they can find each other and build into planets. In the case of accretion disks, the “planets” are black holes - captured single black holes end up in the same migration trap, eventually find each other, form a binary pair, and then quickly merge. This mechanism helps lone black holes find each other, so it should massively boost the number mergers. And boosting the mergers also helps black holes to grow in mass more quickly. Multiple black holes can end up in the same migration trap and merge one at a time, ultimately reaching enormous sizes. This is one of the calculations of Yang and collaborators: they figure that black hole merging in this way should have much higher masses than via the “traditional” empty-space mergers - with 50-solar-mass mergers being relatively common. Another paper by Jillian Bellovary and co. predicts that behemoth “intermediate mass” black holes can form, with 1000s of times the mass of the Sun. 

So all of this sounds exciting and fun - and it may explain why so many surprisingly massive black hole mergers are observed. And if we spot more and more high-mass mergers that will be further evidence in favour of the hypothesis. Yang and co. also predict a particular distribution of black hole spins - again to be tested with more LIGO observations. But what about a more direct test? That’s where the paper by McKernan and collaborators comes in. If black holes merge in empty space then the event should invisible - it should emit no electromagnetic radiation. But it’s different in an accretion disk. A few things happen right after merger that could lead to a bright burst of light to accompany the gravitational wave.

These captured black hole binaries will be surrounded by their own mini-vortices of gas. When they finally merge, they release a burst of gravitational radiation so powerful that it can carry away up to several percent of the original mass of the two black holes. Gas that was orbiting the binary suddenly finds itself moving too quickly for the reduced gravitational field of the final black hole. It expands, creating an expanding shock-front that then collides with the gas of the surrounding accretion disk. Some of the shocked gas will then fall back in, resulting in a burst of accretion onto the new, merged black hole. Finally, the release of gravitational waves delivers a kick to the final black hole - a bit like the recoil of a gun. This drives the black hole and its surrounding gas through the accretion disk, causing more shocks as gas is rammed together. All of this violent motion produces a flash of ultraviolet radiation - and those flashes may be visible to telescopes on Earth right after the gravitational waves arrive.

Now it’s going to be a bit of a challenge spotting these. The combined resolution of the two LIGO and the VIRGO observatories locates a gravitational wave source to a pretty large blob on the sky, which will typically contain hundreds of active galactic nuclei and many thousands of regular galaxies. Any of those could be the source of the black hole merger. But since LIGO started operation we now have advanced follow-up systems in place. As soon as a candidate wave is detected, multiple telescopes scan that region of the sky to search for electromagnetic signatures. If the merger was inside an accretion disk then might see a temporary increase in the light from one of the active galaxies — a fading flash that is brightest at ultraviolet wavelengths.

The researchers are currently scouring the follow-up observations of past black hole mergers for just such a signature, and will be keeping a close eye on future mergers. With LIGO now detected a black hole merger event every week or so, there’s a good chance this will be spotted —- assuming these accretion disk mergers are really happening, and that all the assumptions of the model hold up. 

Anyway, like I said: gravitational wave astronomy will reveal many cosmic mysteries and strange phenomena. Now we have the amazing possibility of black holes merging and growing to enormous sizes while trapped within the blazing vortices around even vaster supermassive black holes. Awesome, sure, but what else would you expect from this supremely badass and frankly, totally metal Space Time.