The Hubble Space Telescope has been one
of our greatest cosmic tools for
decades. We've used it to study
starbursts, black holes, beautiful
nebulae, and much more.
But what it's been particularly useful
for is exploring other galaxies. And
when a team of Hubble scientists led by
Yale University astronomer Peter van
Dokkum were looking at a nearby dwarf
galaxy in 2023, [music]
they noticed something strange, an oddly
straight line of young, blue stars
[music] and gas,
stretching almost twice the length of
our own Milky Way galaxy and containing
the mass of roughly 100 million suns,
this streak of newborn stars is truly
spectacular.
And it was a remarkable find. In fact,
upon making the observation, Van Dokkum
said, "Something like this has never
been seen anywhere in the universe."
The team quickly got to work attempting
to explain this streak of stars. And
they came up with several possibilities.
Could it be a super thin, edge-on,
bulgeless galaxy? Perhaps a runaway
black hole?
Or even jet-induced [music] star
formation?
To work it out, they needed more
information, and thankfully that was on
its way. Follow-up data from the W.M.
Keck Observatory in Hawaii revealed that
the oldest [music] part of the streak
stretched back to a dense, star-forming
galaxy about 7.7 billion light-years
from Earth. The streak itself was nearly
200,000 light-years long.
But these discoveries themselves pose
some serious questions. Where did this
long, thin band of stars come from?
>> [music]
>> And why did it seem to be forming in the
wake of a seemingly massive object?
The discovery team considered all of
their potential explanations for the
streak of stars, but even with the new
data, none of them seemed to fit.
The first hypothesis was that this line
of stars could be a super thin galaxy
oriented side-on to us, so we could just
see the edge of it.
But observations showed that these
weren't regular stars. They were all
young and blue.
A galaxy is usually made of stars that
are different ages and colors. Not only
that, but the front line of stars gave
off an extremely bright signature for
[music] doubly ionized oxygen or O3.
This means two electrons have been
stripped away from each oxygen atom.
Making O3 requires very high energy, and
it only forms [music] in extreme
environments. Yes, galaxies can be
extreme, but the chances of it being so
at one end like this were minuscule.
These observations meant [music] that
the culprit likely wasn't a narrow
galaxy.
The researchers also interrogated the
idea that it could be jet-induced star
formation from the nearby galaxy.
Now, we already have examples of radio
jets triggering star formation. We've
seen it in Minkowski's object, a
peculiar star-forming galaxy near NGC
541, and we've also seen radio-induced
star formation in Centaurus A.
Perhaps that would explain a straight
line of stars.
If this were the case, a radio jet from
within the nearby galaxy would shock gas
in its path, compressing the
interstellar gas and increasing its
density. This could trigger star
formation along the jet's path.
And whilst we do see a shock front, more
on that later, a simple, yet serious,
problem arises with this explanation.
The shape.
This feature is narrowest at the tip,
the furthest point from the nearby
galaxy. In contrast, a jet reaches its
widest width at the furthest point.
Similarly, the strongest interactions
from a jet would be observed [music]
closest to the origin of the jet, the
galaxy, if this theory was right. Yet,
the opposite seems to be true for this
unknown feature.
The more possibilities the team ruled
out, the more they started to wonder
whether this streak of stars could
[music] have a more extreme, elusive
explanation. A runaway black hole.
>> [music] >> back to runaway black holes.
For decades, the idea of a runaway black
hole was purely theoretical. In 1983,
physicist Michael Fitchett showed that
merging black holes could create
asymmetric gravitational waves causing a
powerful recoil that could then send the
merged black hole shooting off in one
direction.
Mathematically speaking, these could
exist.
>> [music]
>> But no one had ever observed one. And
astronomers knew it would be difficult
to spot unless it happened to create a
large visible footprint, that is.
The answer came a few years after the
streak's initial discovery when in
December of 2025, Ann Dookum once again
led a team of astrophysicists in
exploring this space oddity, this time
using the James Webb Space Telescope.
With its better resolution, the team was
able to observe and measure the feature
in greater detail.
One of the most important pieces of
evidence confirmed by the James Webb
Space Telescope was that there was an
absolutely enormous bow shock at one end
of the line of stars.
Just as air is compressed in front of a
supersonic jet or a boat pushes through
the water and
>> [music]
>> something was pushing on the
interstellar medium.
Now, obviously, this bow shock is not
being created by a plane or a boat. The
team thought the most likely culprit was
actually a supermassive black hole, at
least 10 million times the mass of our
sun, as it plows through space [music]
at roughly 1,000 km per second,
compressing heated gas in front of it.
But, they still had to prove it.
They were able to measure strong
emission lines that confirm the gas at
the very tip of the structure had been
heated and compressed by a powerful
shock.
They also found a sudden jump in gas
velocity, indicating that something is
moving extremely fast, supersonically in
fact, through space.
All of this evidence pointed to a single
conclusion. The entire structure must be
the wake of something absolutely
massive.
The only thing massive and compact
enough to have this effect would be a
runaway supermassive black hole.
So, the team had confirmed the first
photographic evidence of a runaway
supermassive black hole, and they named
it, inventively, as
Runaway [music] Black Hole One, or RBH1.
But, there's one problem with this
discovery.
We typically think of a black hole as
the source of a violent destruction,
capturing nearby stars and solar
systems, even light itself, until it has
devoured everything in its path.
How is it that this black hole is
creating stars? [music]
As RBH1 carves a chaotic path through
space, its powerful bow shock
>> [music]
>> causes compressed gas to accumulate and
trail behind.
As it cools and mixes with other
surrounding gases, clumps along the
trail of gas collapse to form new stars.
It is the very chaos of this runaway
supermassive black hole that ultimately
seeds the distant cosmos with the matter
and conditions [music] to form new
stars.
From cosmic carnage comes stellar
rebirth.
But why is RBH1
hurtling through intergalactic [music]
space at breakneck speeds in the first
place?
Where did it come from?
Luckily, this part of the mystery comes
with its own line of evidence,
literally.
The team led by Van Dokkum said that
with the black hole's current velocity
of about 954 km per second, the evidence
is very strong, bordering on
overwhelming, that this is indeed a
runaway supermassive black hole that was
sent traveling from its once host
galaxy.
It was flung fast enough to escape its
galaxy and eventually also the
circumgalactic
medium, the faint halo of gas that
surrounds the galaxy.
In RBH1's case, the stellar vein leads
right back to the source, the cosmic
owl.
This two-galaxy system is about 7.5 to 9
billion light years away, and it got its
name because it looks like an owl with
two eyes and a beak.
This unique galaxy offers clues to what
sent [music] RBH1 on its current
trajectory.
The shape of this two-galaxy system is
very unusual and was created through the
extremely rare collision of two ring
galaxies.
Ring galaxies make up just 0.01%
of all the galaxies we've discovered so
far with only a few hundred known in the
local universe.
Why?
Well, in order to form, they require a
galaxy to pass almost directly through
the center of another galaxy.
As you can imagine, that's not easy.
Just like if you're playing darts,
chances are much greater that the dart
will land off-center, no matter how good
your aim, than directly on the
bull's-eye.
And well, galaxies don't come with an
innate sense of aim.
This means the chance of one ring galaxy
being created is already rare. So, two
of these rare galaxies existing close to
each other is even more unusual.
Take that one step further and the
chance that two rare ring galaxies
collide with each other,
well, you can see why the Cosmic Owl,
also dubbed the Infinity Galaxy, is an
exceptionally rare type of galaxy
merger.
Astronomers say that the symmetry of the
resulting Cosmic Owl is a strong
indicator of a head-on collision between
two galaxies of similar mass.
Both of the ring galaxies that make up
the Cosmic Owl are relatively small,
each measuring in at only about a
quarter of the diameter of the Milky
Way.
At the time of the collision, each of
the Owl's eyes represented a galactic
core of densely packed [music] old stars
surrounding a black hole.
Where these eye-like galaxy edges began
to merge some 38 million years ago, we
get the area known as the beak.
This is a place of active collision
packed with molecular gas, the fuel for
star formation.
This ancient collision may have been the
very catalyst that sent RBH 1 on its
rapid escape path.
There are two leading explanations for
what could have happened to RBH 1.
Either a merger of two black holes or a
three-body interaction with a third
black hole involved. So, which is it?
When ring galaxies like these collide,
the supermassive black holes spiral
nearer and nearer to each other, drawn
together by their massive gravity. When
the observation [music] was initially
revealed in 2023, before it had been
confirmed by the Webb, the leading
explanation [music]
seemed like it could be the three-body
gravitational interaction scenario,
where three supermassive black holes
were brought close enough to interact
with each other through the galactic
merger.
In this case, the two supermassive black
holes at the galactic cores of the
merging galaxies spiral together to form
a binary black hole system at the
center. This kind of binary black hole
system [music] can last billions of
years, and if a third supermassive black
hole gets close enough to the galactic
[music] core, this three-body
interaction can send one of them flying
out of the core with enough velocity
[music] to escape the galaxy altogether.
But with more accurate measurements, it
became clear that the better fitting
scenario was actually a black hole
merger.
The mass was the smoking gun in the
mystery. If it had been three black
holes interacting, [music]
the lowest mass black hole would be the
most likely one to be shot out of the
system, whereas in a merger, the larger
combined black hole is the one that
would be ejected. In this case, the
supermassive black hole's mass was
calculated to be greater than 10 million
times the mass of our sun, too massive
to be a small third black hole, but just
what would be expected from a binary
merger and gravitational kick.
As the two ring galaxies of the Cosmic
Owl collided, the supermassive black
holes at their respective galactic cores
merged [music] to form one new
supermassive black hole. The
asymmetrical release of gravitational
waves from their merger would have
caused such a violent recoil that it
would easily be enough to suddenly send
the new black hole flying out of the
host galaxy.
Models of this scenario are consistent
with measurements of
and the mass of the galaxy it left
behind. In other words, this theory is
consistent with what we've measured and
simulated. If the team is correct, the
ring galaxies' central black holes would
have merged into one supermassive black
hole that was flung out of the Cosmic
Owl by the gravitational wave recoil.
In a merger like this, the galaxy system
may be temporarily left with no central
black hole, although we don't have
concrete evidence one way or the other
just yet.
Whilst this remains a mystery, we do at
least know that this unusual galactic
merger sent a supermassive black hole on
a race toward intergalactic space.
The confirmation of RBH 1 marks a new
era in our understanding of how
supermassive black holes, galaxy
mergers, and intergalactic star
formation can relate to each other.
This is the first clear observational
confirmation that supermassive black
holes can be slingshot out of their host
galaxies, and it has revealed the
stunning side effect of leaving a string
of young stars in its wake.
Before the observation, simulations had
predicted the wake of a runaway black
hole might glow, but few expected them
to become stellar nurseries.
Even more excitingly, if a runaway
supermassive black hole can happen once,
it can happen again. After all, galaxy
mergers are common and happen often over
the lifetime of a galaxy. So, black hole
binaries should happen regularly, laying
the way for black hole mergers or
three-body interactions.
For a long time, we've seen black holes
as permanent fixtures within their host
galaxies, but now it seems this is
certainly not the case.
Even before this confirmed event,
astronomers had spent years gathering
evidence of other runaway supermassive
black hole candidates.
And we think we found multiple instances
of wandering black holes, like this
spiral galaxy, the memorably named
J0437-2456,
located around 288 million light-years
from Earth.
It's about 3 million times the mass of
our sun and seems to be moving at a
significantly different velocity
compared to the rest of its host galaxy.
In another case, the central galaxy in a
cluster known as Abell 2261 should have
one [music] of the biggest supermassive
black holes in the entire universe,
between 3 and 100 billion times the mass
of our sun.
And yet, [music] it seems to be missing
without a trace.
A team of astronomers are investigating
the hypothesis that, similar to with RBH
1, this black hole may have been kicked
out of its host galaxy due to
gravitational recoil.
These curious cases confront researchers
with even more questions.
How often are supermassive black holes
flung from their galaxies? And how many
wandering or runaway black holes are out
there zooming through intergalactic
space?
Despite other candidate runaways, RBH 1
is, for now at least, still the only one
we've been able to confirm. And it was
difficult to spot, just a thin, faint,
far-away signature.
That could be about to change with the
Euclid mission and the Nancy Grace Roman
Space Telescope's wide scans of the sky
in Hubble-like clarity.
And with the introduction of machine
learning tools to help spot these long
streaks, we may go from just one
confirmed case to an entire population
of known runaway supermassive black
holes.
For now, one thing is for certain,
galaxy collisions may last for billions
of years, but they are anything but slow
or peaceful events. Marked by chaotic
collisions and violent forces, they're
capable of launching entire supermassive
black holes on a runaway path across
intergalactic space.
How odd and beautiful it is that these
destructive scars across space are the
very reason that millions of new stars
are capable of being born in their wake.
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