This video is sponsored by Magellan TV.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood astronomer.
We've talked a lot about the hunt for intermediate mass black holes that should exist in the
mass range between stellar mass black holes that are formed in supernova explosions and
supermassive black holes that dominate the centers of galaxies.
These middleweight black holes have eluded our detection until fairly recently. In the
spring of 2020, astronomers used the Hubble Space Telescope to pin down an intermediate
mass black hole in the core of a dwarf galaxy by watching it shred apart a passing star.
Later that same year, another team announced the detection of the gravitational waves given
off by the merger of two black holes into an intermediate black hole.
But so far, intermediate mass black holes haven't been found within our own Galaxy.
So astronomers are searching for these middleweights in the cores of globular star clusters. They
do this by analyzing the motions of stars in the cluster to determine the mass and size
of whatever lies at the very center.
But when a pair of astronomers analyzed the motions of stars in the globular cluster NGC
6397, they found that the stars seem to be moving around what appears to be a graveyard
of stellar black holes!
This finding is exciting because it has implications not just on how intermediate mass black holes
might form, but also for how globular star clusters evolve. So we're going to break all
of this down and understand why a sub cluster of black holes might be lurking in the core
of NGC 6397, but first I'd like to thank Magellan TV who are very kindly sponsoring today's
video.
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Their documentary on Monster Black Holes talks about how supermassive black holes are thought
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Intermediate-mass black holes are one of the holy grails in black hole research because
there's no theoretical reason why they shouldn't exist, and they could also explain some unresolved
mysteries in astrophysics. For example, they could account for some of the mass of dark
matter. They could also explain how supermassive black holes were able to form so early in
the Universe's history.
So, astronomers have been searching for modern-day intermediate black holes in the cores of globular
star clusters.
These are spherical clusters of hundreds of thousands of stars held together by their
mutual gravity. They're relatively small at less than 100 light-years across. That makes
for a crowded ball of stars swarming around like bees in a beehive.
When any two stars zip past each other, the subtleties of gravitation, energy conservation,
and motion come into play; the more massive of the two sinks closer to the center of the
cluster while the lighter star gets flung to the outer regions. Over time, the core
becomes populated with massive stars and the outer envelope expands with lighter, faster-moving
stars.
This process is called mass segregation and under the right conditions, this can happen
very rapidly and the result is a core-collapsed globular cluster. About one in five globulars
are of this core-collapsed variety, and it's thought that intermediate mass black holes
could form in these cores by a variety of mechanisms. For example, massive stars in
the core could go supernova and their black holes merge together. Or perhaps there were
already several black holes in the cluster that sank to the core before merging. It's
even possible that there were just a bunch of massive stars that sank quickly to the
center and merged together before the super "megastar" could ever go supernova.
At least those are the predictions of some models. But there are others that leave the
door open for another possibility: perhaps instead of a single large black hole, there
could be an inner sub-cluster of smaller black holes.
Now, it would be great if we could point Hubble or James Webb at a cluster and count the black
holes in the center. But black holes are just too small on the sky to be resolved by telescopes.
Yes, M87's black hole is the exception, but it's also the largest black hole in the sky
and it took a virtual telescope the size of Earth just. to image it. Even a thousand solar
mass black hole in a nearby cluster is going to be far too small to directly image.
So instead, astronomers analyze the motions of its stars as they swarm about, and infer
the mass and size of the central object.
Now, NGC 6397 is only 7800 light-years away, making it the second-closest globular cluster
to Earth. That's close enough to actually see the stars move with the Hubble Space Telescope
and Gaia satellite. Even better, it's a core-collapsed globular so it's prime hunting grounds for
intermediate black holes.
But so far, the cluster has produced mixed results because there are limitations on what
kind of motions can be detected at any given time.
For example, the line of sight motions of stars, or motions toward or away from us,
can be measured relatively easily by measuring their individual doppler shifts. And in 2016,
Kamann et al used the Very Large Telescope to work out the line of sight motions of stars
within the cluster. They found that a single black hole weighing in at 600 ± 200 solar
masses could account for the stars' line of sight motions.
That puts the candidate black hole firmly in Intermediate mass territory.
However, in 2017, H. Baumgardt showed that an intermediate black hole was unlikely to
exist in NGC 6397 because the distribution of the stars in the core didn't match what
was predicted by models containing intermediate-mass black holes. However, it left the door open
for another possibility; there could still be a sub-cluster of densely compact massive
objects. In other words, stellar black holes.
But over the last decade, the cluster has been observed repeatedly by both Hubble and
Gaia. So, Gaia can observe the stars [in the cluster's] outer envelope could and trace
their motions very precisely, while Hubble can trace the motions of the stars in the
cluster's core. So two astronomers from of the Institut d’Astrophysique
de Paris combined the proper motion data from Hubble & Gaia, with the line of sight motions
from the Very Large Telescope to work out the true motions of stars in NGC 6397.
However, a decade is not really a tremendous amount of time, and that's why the proper
motions that they do have are very tiny and therefore have a lot of uncertainty in them.
So that required some new statistical modeling to work out different scenarios for the cluster
and see which scenario for the cluster's interior best matches the data we do have.
Vitral and Mamon ran a series of simulations on three different cluster configurations.
Some runs included an intermediate-mass black hole, others an inner cluster of unseen objects,
and finally simulations without anything compact in the center were run as a check on the other
two. All three of these model simulations were then compared against the Gaia, Hubble,
and VLT data.
As it turned out, the models that lack a central object didn't match the data very well at
all. So, it's pretty clear this cluster holds something pretty massive at its center. The
question now is "what, exactly"?
Well, the results don't completely rule out an intermediate mass black hole. Models with
500−650 solar mass black holes in the centers matched most of the data fairly well.
But much better fits were made with models that have an extended sub-cluster in the core.
Specifically, between 1000-2000 solar masses crammed into a spherical volume about half
a light-year across.
Think about that just for a second - Alpha Centauri, the closest star to the Sun, is
4.3 light-years away. But this cluster requires at least a thousand Suns worth of stuff to
be crammed into just one-half of a light-year!
The model predicts that stellar mass black holes account for about 58% of the mass in
this region, with about 30% coming from white dwarfs, and roughly 12% from neutron stars.
However, the black holes may not dominate the core's mass forever. Some of them could
merge, in which case about 5% of the merging mass would be converted into gravitational
waves. Those waves can conceivably kick the merging pair into motion. If the waves are
strong enough, the merged black hole can kick itself out of the cluster!
Another possible scenario anticipates that the most massive black holes of the sub cluster
could reach the center of the cluster first and merge. So it's possible that this could
end up in some weird mix of a very massive-yet-not-intermediate-mass black hole at the center, surrounded by smaller
black holes swarming around it. In other words, a globular cluster of black holes inside a
globular cluster of stars!
For now, it looks like this is our first detection of a black hole graveyard that may or may
not form an intermediate mass black hole someday.
But, the cluster will need to be monitored over a longer timescale to watch those stars
move and better trace out their proper motions. That will allow astronomers to better characterize
the nature of the cluster's core.
Luckily, the Gaia mission just announced their third data release, which includes additional
measurements of this cluster. And, there's another big data release on the way! Gaia
is the most amazing astronomy mission that most people haven't heard of, which is why
I made a video about it that I invite you to check out.
And did you know that astronomers detected the merger of two large black holes merging
into an intermediate mass black hole by their gravitational waves? I made a video about
that which I invite you to check out as well!
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