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[♪ INTRO]
Black holes are one of the most mysterious phenomena the cosmos has to offer.
I mean, the idea of cramming unfathomable mass
into infinitesimal space literally takes physics to its breaking point.
And, yet, somehow, that might not even be the most mysterious thing about them.
That’s because, while physicists can describe the properties
of a black hole pretty well, they’re still struggling to get to the bottom of
how the biggest, baddest ones form at all.
That didn’t change after the recent publication of two new studies.
Each added new details to the origin story of supermassive black holes,
but, in the end, there’s a lot that scientists are still in the dark about.
The first of these papers, published November 5th in The Astrophysical Journal,
reports the surprise discovery of a new supermassive black hole
located right next door to us.
And here’s the kicker: the black hole might be gigantic,
but its host galaxy is anything but.
The galaxy is named Leo I and it’s located in orbit around the Milky Way,
about 840,000 lightyears from Earth.
As a so-called dwarf spheroidal galaxy,
it’s relatively small with a symmetrical shape.
Given its relative closeness, Leo I has been a popular target for astronomers
over the years, but it’s been difficult to peer into its densely packed core.
That’s the breakthrough made by this study, which made use of a one-of-a-kind
instrument installed on the Harlan J. Smith Telescope
at the McDonald Observatory in Texas.
That instrument allowed the team to track the motion of
stars near the galaxy’s center.
They combined their new measurements from Leo I’s core with older observations
from the rest of the galaxy, and fed all that information
into a supercomputer simulation.
Because a star’s speed around the galactic core is directly linked to
how much mass is near the center, the simulation was able to use the motion of
all the stars to work out the size of the central black hole.
And the result was kind of a shocker.
The computer’s best estimate of its mass was around
3.3 million times larger than the mass of our Sun.
That’s around three-quarters the size of the supermassive black hole at the center
of the Milky Way, even though Leo I is around 30 times smaller than our galaxy.
So how did such a small galaxy get such a big black hole?
Scientists don’t really know.
But they don’t really know how any supermassive black holes form,
so this is just one more piece of the puzzle.
Yet another piece of that puzzle was revealed through new work published
November 22nd in the journal Astronomy & Astrophysics.
The paper describes one of the rarest known black hole configurations:
a single galaxy hosting not one, but two supermassive black holes.
What makes this discovery especially exciting is that the galaxy, called NGC 7727,
is around five times closer to us than any
previously known double-black hole galaxy.
Of course, “close” is relative in space.
This object is around 88 million lightyears from Earth.
Astronomers have known about NGC 7727 for quite a while,
but it took the European Southern Observatory’s Very Large Telescope
to zoom in enough to make this discovery.
That’s because these two giant black holes are incredibly close to one another,
separated by only about 1600 lightyears.
The larger of the two is truly ginormous,
weighing in at over 150 million solar masses.
The smaller one, and, again, size is very relative here,
is around six million times heavier than the Sun.
That means that even the smaller black hole is around
50% more massive than the one at the center of the Milky Way.
Another reason that this discovery is only being made now is that this galaxy isn’t
emitting the kinds of high-energy radiation that scientists
usually expect to come from supermassive black holes.
That could also be true for other galaxies, suggesting that there could be
many more of these objects than astronomers previously expected.
So how did such a strange situation arise?
The evidence is clear that NGC 7727 is the result of two spiral galaxies
merging together in an eons-long collision.
Eventually, the two black holes will merge as well,
which may be a common way for these giants to grow.
In fact, there’s a good chance that’s how the
weirdly large black hole at the center of Leo I got there.
Long ago, the Milky Way may have had
many more satellite galaxies than it does today.
Some of those could have been eaten by Leo I
through a series of unfortunate encounters.
Well, unfortunate for those galaxies.
But for astronomers trying to understand how supermassive black holes grow
and evolve, it’s just the kind of disaster they’re looking for.
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