Black Hole Swarms (Script) (Patreon)

It’s been conjectured that the center of the Milky Way is swarming with tens of thousands of black holes. And now we’ve actually seen them.

The core of our galaxy is a wild place. The stars are so densely packed that the night sky would be 500 times brighter. A supermassive black hole, 4 million times the mass of our Sun lurks in the center. It flings nearby stars into extreme slingshot orbits. It consumes anything that gets too close is consumed, burping a blast of X-rays.  We know these things because we see them from our comfortable vantage point 28,000 light years out in the galactic disk. But  there’s one particularly terrible feature of the core of our galaxy that, until now, has only been has only be hypothesized: The central few light years of the Milky Way is thought to contain a vast swarm of smaller black holes that have rained in from the surrounding galaxy.

In this episode of Space Time Journal Club we’re going to delve into the recent Nature paper Hailey et al. 2018, titled “A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy”. In it, these astrophysicists find powerful evidence that our own Milky Way’s core is packed with 100s, maybe 1000s of black holes.

I’ll get to how they found the black holes in a minute, but first I want to ask, why did many astrophysicists already believe there must be a swarm of black holes in the galactic core? Well, the simple answer to that in straight forward. Dense things sink. Colder, and hence denser water or air sinks to the bottom of the ocean or the atmosphere. Dense elements link iron sink to the centers of planetary bodies as they form in a process called differentiation. And the densest stellar objects sink to the centers of galaxies or star clusters. We think black holes MUST gradually sink to the center of the Milky Way, although the process is a wee bit more complicated. Let me explain.

Black holes form when the most massive stars end their lives in spectacular supernova explosions. After blowing off their outer layers, if the remaining stellar core is massive enough it will collapse into a black hole. We've discussed this whole process in an earlier episode. We expect these so-called stellar-mass black holes to weigh in at between 5 and 15 solar masses, although the recent gravitational wave signals detected by LIGO suggest they may be even more massive. Even after blowing off most of their mass in a supernova, these black holes are still heavier than most stars. This means they migrate inwards by a process called dynamical friction.

It works like this: As a black hole orbits the galaxy, it tugs on its neighboring stars. Those stars are accelerated towards the black hole and can gather behind the black hole in a gravitational wake. That overdensity behind the black hole pulls it backwards, reducing its speed. The black hole can also slingshot stars outwards, losing momentum in that process too. The key is that the more massive object tends to donate its momentum to the less massive object. The ultimate result is that the black hole slows down and no longer has the velocity it needs to maintain its orbit. Gradually it falls towards the galactic center.

Now this process takes a really long time for a stellar-mass black hole. Over a few billion years, we only expect the black holes from the central several light years to have made much progress. However there’s another process that can really drive a huge number of black holes inward. Our galaxy is surrounded by these things called globular clusters. These are like ancient, extremely dense mini-galaxies containing millions of stars. Some are nearly as old as the universe itself. They exist in a swarm surrounding the Milky Way, but sometimes they are captured by the Milky Way and are dragged to its center by this dynamical friction process.

Because globular clusters are much more massive than a single black hole, they reach the galactic center a lot more quickly. Over the life of the Milky Way they have piled up in the galactic core forming a giant nuclear star cluster. Those globular clusters must have been full of ancient black holes, which would be carried to the core with their parent cluster. Those black holes should then  sink ever further to the center of the galaxy. Prior to this new result, it had been calculated that this process should lead to 10s of thousands of black holes in the central few light years of the Milky Way’s core.  

So how did Hailey and team spot these black holes? They’re supposed to be black, no? That’s true: black holes are effectively invisible. But things can be different if a black hole and a companion star are in a binary orbit with each other. If the companion star gets to close, its outer regions can fall into the gravitational influence of the black hole. Gas is syphoned off the star into whirlpool – an accretion disk – around the black hole. That gas heats up to crazy temperatures. To us it looks like a range of heat-glows - thermal radiation at different temperatures, with the hottest glowing with extremely energetic X-rays. These X-ray binaries are seen throughout our galaxy.  By the way, X-ray binaries can also result from a neutron star rather than a black hole cannibalizing its companion, but today we’re interested in black holes.

The brightest X-ray binaries are aggressively gobbling up their companion. But that ravenous phase probably doesn't last all that long. X-ray binaries likely spend most of their time in a quieter phase, with the gas trickling only slowly from the companion star. These quiescent X-ray binaries should be seen much more frequently than the active ones. Frequently enough that if the galactic core is full of black holes then it should also contain quiescent X-ray binaries.

Hailey et al. used the orbiting Chandra X-ray observatory to hunt for these. And, surprise surprise, they found them. They spotted 92 point-like X-ray sources within one parsec, or around 3 light years of the galactic center. These were potential X-ray binaries, but there are other astrophysical critters that also shines bright in X-rays. One we expect to be common in the galactic core are magnetic cataclysmic variables – also called polars. Polars are a bit like X-ray binaries, except instead of a black hole or neutron star you have a white dwarf with a powerful magnetic field. Those magnetic fields act sort of like a dam, allowing gas to build up and then fall very suddenly onto the white dwarf producing a burst of X-rays. But these polars produce a very different spectrum to X-ray binaries.  Polars only glow at a single extremely high temperature, while X-ray binaries glow at both high and low energies due to the large temperature range of the accretion disk. That allowed the researchers to weed out the X-ray sources that had the wrong spectra.

After weeding out polars and other uninteresting sources, there remained 13 probable quiescent x-ray binaries, which appeared to be the type powered by black holes. 13 doesn’t sound like a swarm – but remember, only a small fraction of black holes are seen as X-ray binaries. The researchers extrapolate that there would need to be at least hundreds of stellar-mass black holes in the central few light years in order to get these 13 X-ray binaries. That’s tens of thousands times the black hole density anywhere else in the galaxy. So yeah, it’s a swarm of black holes. If the sun was near the galactic core, the nearest black hole would be inside the solar system’s Oort cloud.

Besides being very cool and kind of freaky, this result is especially important for the new field of gravitational wave astronomy. We keep seeing the gravitational wave signals of these black hole mergers, and as we’ve discussed previously, they’re kind of confusing. If black holes are so densely packed in the centers of galaxies, then we should probably know that if we want to understand the source of these gravitational waves.

So next time you see the Milky Way on the night sky, find the bright patch just to the edge of the constellation of Sagittarius. Consider what lies beyond that dusty veil. Not just one gigantic back hole, but also a swarm of hundreds, perhaps many thousands of smaller black holes, in what has to be the craziest and more terrifying environment in nearby space time.