[MUSIC PLAYING] It's been conjectured that the center of the Milky Way contains not one, but a vast swarm of black holes, and now, we've actually seen them.

[MUSIC PLAYING] The core of our galaxy is a wild place.

The stars are so densely packed that the night sky would be 500 times brighter than our own.

A supermassive black hole-- four 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, burping out blasts of x-rays.

We know these things because we see them, from our comfortable vantage 28,000 light years out in the galactic disk.

But there's one particularly terrible feature of the core that has only been hypothesized until now.

The central few light years of the Milky Way is thought to contain a vast swarm of smaller black holes that have reigned 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 Quiesce X-ray Binaries in the Central Parsec of the Galaxy."

In it, these astrophysicists find powerful evidence that our own Milky Way core is packed with hundreds, maybe thousands, of black holes.

I'll get to how they found these black holes in a minute, but first, I want to ask, why did so many astrophysicists already believe there must be a swarm of black holes in the galactic core?

Well, the simple answer is straightforward-- dense things sink.

Colder and hence denser water or air, sinks to the bottom of the ocean or the atmosphere.

Dense elements, like iron, sink to the centers of forming planetary bodies in a process called differentiation.

And the densest, stellar objects, like black holes, 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 exact 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'll collapse into a black hole.

We've discussed this whole process in an earlier episode.

We expect the so-called stellar-mass black holes to weigh in at between five and 15 solid amasses, 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 towards the center of the Milky Way, in 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 it in a gravitational wake.

That over-density behind the black hole, pulls the black hole 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-- usually, the black hole-- 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 circular 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 inwards.

However, there's another process that can really drive a huge number of black holes inwards.

Our galaxy is surrounded by these things called globular clusters.

They're like ancient, extremely dense mini-galaxies, containing millions of stars.

Some nearly as old as the universe itself.

They exist in a swarm surrounding the Milky Way, but sometimes, they're captured by the Milky Way and 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 will quickly.

Over the life of the Milky Way, they have piled up in the galactic core, forming a giant nucleus 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 would then, sink even further to the center of the galaxy.

Prior to this new result, it had been calculated that this process should lead to tens 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?

Well, 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 around each other.

If the companion star gets too close, its outer regions can fall into the gravitational influence of the black hole.

Gas is siphoned off the star into a 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 the 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 stars, but that ravenous phase probably doesn't last all that long.

X-ray binaries likely spend most of the time in a quieter phase, with the gas just trickling slowly from the companion star.

These quiescent X-ray binaries should be seeing 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 and team 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 three light years, of the galactic center.

These were potential X-ray binaries, but there are other astrophysical critters that also shine 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 a neutron star, you have a white dwarf with a powerful magnetic field.

Those magnetic fields act like a dam, allowing gas from the companion star 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 allow 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.

Now, 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.

Now, that's tens of thousands of 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 systems Oort cloud.

Besides being very cool and kind of freaky, this result is especially important for the new field of gravitational wave astronomy.

Now, we keep seeing these gravitational wave signals from black hole merges, and as I'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 in 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 black hole, but also, a swarm of hundreds, maybe thousands, of smaller black holes, in what has to be the craziest and most terrifying environment in nearby spacetime.