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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.
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, 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, 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 that has only been has only be
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 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 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 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
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 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 wave.
That overdensity 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.
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?
I mean they're supposed to be black, no?
While 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 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.
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 just trickling
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 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 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 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 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.
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
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 from 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 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 space time.
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Last week we talked about some of the incredible ways for detecting gravitational waves beyond LIGO.
You guys had a lot to say.
Majestic Potato asks whether supernovae can produce gravitational waves detectable from earth.
Actually, yes, and LIGO itself may be able to see them.
The trick is that the supernova can't be spherically symmetric.
Gravitational waves are produced when the quadrupole moment of a mass distribution changes.
In non-technospeak - they're created in non-spherically or circularly symmetric movement of mass.
So if the explosion of a supernova is concentrated, say, more on one side, then LIGO could potentially
see the resulting gravitational waves.
JuxtaposedStars asks whether, theoretically, you could build an engine to extract power
from gravitational waves via the "sticky bead" method.
Sure, for the right definition of theoretically - that is, the laws of physics allow it.
The laws of engineering may beg to differ.
You'd need a phenomenal amount of matter spread over a vast region.
There may be more efficient ways to gather energy at that scale, like a good old fashioned
Dyson sphere.
A couple of you asked whether gravitational waves interfere with each other, or could
even be used in a two-slit experiment.
The answer to the first is definitely yes - two gravitational waves crossing paths will
add together.
At any one point in space and time this is either constructively, producing a stronger
stretching or contraction of space, or destructively, meaning their effect cancels out.
And the two-slit experiment?
In principle yes, but you'd need a material capable of blocking gravitational waves.
We now know that they can lose energy to matter... but you would need a LOT of matter.
I don't know, maybe a cosmic-scale wall of neutron stars with two gaps in it.
And what would you see?
Well to answer that I'd need a theory of quantum gravity.
So, let me get back to you.
The Rogue Wolf notes that the stellar gravitational wave detectors like the pulsar timing arrays
are a bit like using the rustling of leaves and grass to see the wind.
I don't have anything to add to that.
Sir, you are a poet.
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