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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.
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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.
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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 asked, whether a supernova
can produce gravitational waves detectable from Earth?
Actually, yes, and LIGO itself may be able to see them.
The trick is that supernova can't be spherically symmetric.
Gravitational waves are produced when the quadrupole moment
of a mass distribution changes.
In non-techno speak, they're created
in non-spherically or circularly symmetric movements 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.
Juxtaposed stars 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 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 the gravitational waves
interfere with each other?
Or could even be used in a two-slit experiment?
Well, 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--
well, in principle, yes.
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 stellar gravitational wave
detectors, like 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.