This video is sponsored by Stellarium Mobile
Plus. Made by the original creator of the
award-winning Stellarium desktop planetarium,
Stellarium mobile puts 1.6 billion stars,
and 2 million nebulae and galaxies at your
fingertips. Planets are rendered in high-resolution
textures and the night sky can be viewed in
beautiful 3D in real-time from any location
on Earth. I’ll tell you a little more about
it at the end of this video.
The black holes we’ve detected so far fall
into two categories. Stellar-mass black holes
are often found in binary systems with masses
equivalent to 10 Suns. Then there are the
supermassive black holes, which dominate the
centers of galaxies and weigh millions to
billions of solar masses.
Somewhere between these extremes must be intermediate-mass
black holes with masses ranging from hundreds
to hundreds of thousands of Suns. Models predict
they should reside in the centers of dwarf
galaxies and possibly globular clusters, but
so far, they’ve avoided detection.
But now after years of investigation, a team
of astronomers using the Chandra X-ray, XMM
Newton, and Hubble Space Telescopes made the
most convincing detection yet of the missing
link of black hole evolution. A 50,000 solar
mass black hole, devouring a star a long time
ago in a galaxy far, far away…
Welcome back to Launch Pad, I’m Christian
Ready, your friendly neighborhood astronomer.
Finding black holes is kind of hard to do
because they’re…black. Not dark, black.
They greedily consume light, but never radiate
so much as a single photon back into space.
The only way we can detect black holes is
by detecting the radiation coming from just
outside their event horizons.
For example, we’ve detected stellar mass
black holes in binary star systems. If the
two objects are close enough, matter can flow
from the companion star into an accretion
disk around the black hole. The disk feeds
the black hole but as it does so, it heats
up to millions of degrees, producing X-rays.
Cygnus X-1 was the first black hole discovered.
It’s caused by a blue supergiant star losing
mass to a 15 solar mass black hole. We’ve
detected several stellar mass black holes
this way, both here in our Galaxy as well
as in nearby galaxies.
Then there are the supermassive black holes
that live in the centers of galaxies, including
our own. Sagittarius A* holds court in the
center of our Milky Way Galaxy. At 4 million
solar masses, it occasionally feeds on clouds
of gas, generating X-ray and radio emission
that penetrates the spiral arms that would
otherwise block its view.
The black hole at the heart of M87 was initially
revealed by a 5000 light-year long jet of
plasma hurtling toward us at nearly the speed
of light. Then it was imaged directly by the
Event Horizon Telescope. This black hole is
6.5 billion times the Sun’s mass!
Despite their extreme differences in mass,
all black holes follow a simple rule; the
greater their mass, the larger their event
horizons. For example, to make a 50 solar
mass black hole, you’d need to squeeze 50
Suns into a ball less than 300 kilometers
across (200 mi). M87’s black hole is the
equivalent of 6.5 billion Suns squeezed into
a ball about the size of our solar system.
But if the Universe can have supermassive
black holes, then surely there must be intermediate
mass black holes out there, right? Not necessarily.
Nature is under no obligation to make sense
to us.
However, it seems that intermediate black
holes are needed to solve a problem with supermassive
black holes at the centers of the earliest
galaxies.
These black holes consume enormous quantities
of gas and their accretion disks shine with
the light of up to a trillion Suns. They’re
so bright we can see them from across the
Universe as “quasars”. Because of light’s
finite speed, we see quasars when the Universe
was less than a billion years old.
The most distant quasar known is ULAS J1342+0928.
The quasar emitted its light when the Universe
was less than 690 million years old, yet its
supermassive black hole is estimated to be
800 million solar masses.
So did a nearly billion solar-mass black hole
form just a few hundred million years after
the Big Bang?
At first it might seem that it started out
as a stellar mass black hole that simply grew
as it fed on the surrounding matter.
But there’s a problem with this hypothesis.
Remember that accretion disk we talked about?
It puts out a lot of radiation. So much that
it exerts a strong pressure on the surrounding
disk, pushing it outward. Some of it still
manages to find its way into the black hole,
but at more of a trickle than a deluge.
Therefore, it might take a stellar mass black
hole up to 30 million years to double its
mass through normal accretion. Even 700 million
years isn't enough time for a supermassive
black hole to grow from such a small seed.
In order to create supermassive black holes
while the Universe was still young, they would
have had to grow from much larger seeds. Black
holes in the hundred to hundred-thousand solar
mass range.
Fortunately, the conditions in the very early
Universe allowed such black holes to form
pretty quickly.
A couple of hundred million years after the
Big Bang, the Universe was a dark cloud of
pure hydrogen and helium gas. With no heavier
elements present to arrest its flow, hydrogen
was free to rapidly collapse into ultra-massive
stars that weighed as much as a thousand Suns.
These stars would explode as hyper novae after
just a couple of million years, producing
violent gamma ray bursts as their cores collapsed
into 100 solar mass black holes.
Some of those first stars got so massive,
they skipped going supernova altogether and
their entire mass imploded into a several
hundred solar mass black hole!
In fact, some of those hydrogen gas clouds
may have collapsed so quickly, they may have
collapsed directly into thousand solar mass
black holes without ever forming a star!
Such black holes would have been born into
a buffet of hydrogen, helium, and nearby stars.
Soon they’d occupy the centers of the first
protogalaxies which would quickly merge with
others. Their central black holes would soon
find each other and merge into the supermassive
black holes we see today, while entire galaxies
grew around them.
But not all of these intermediate black holes
would merge. Countless protogalaxies exist
today as dwarf galaxies, presumably with intermediate
black holes hiding in their centers.
However, most dwarf galaxies are devoid of
gas, so their black holes are less likely
to continuously feed and emit radiation.
But if we’re lucky, we can catch a black
hole shredding a passing star.
If a star comes too close to a black hole,
tidal forces increase until the star can no
longer hold itself together. The star shreds
and encircles the star, heating up to hundreds
of millions of degrees and unleashing a torrent
of X-ray radiation.
This is a Tidal Disruption Event or TDE. TDE’s
have been detected in our own Galaxy, allowing
astronomers to discover new black holes.
These events are rare, occurring only once
every 10,000-100,000 years in our Galaxy.
But they create Hyper-Luminous X-ray events
that can emit up to a billion times more X-ray
energy than the Sun. That’s bright enough
to be detected in distant galaxies. If we
can catch one of those events, we may be able
to determine if a black hole was involved,
and weigh it.
TDEs have a characteristic light curve of
an almost immediate outburst that holds steady
for a couple of years, followed by a long,
gradual decline in X-rays over time.
Several intermediate black hole candidates
have been detected in dwarf galaxies and even
in some globular clusters from X-ray events
that were thought to be due to TDEs. But as
we’ll see, the X-ray light curve produced
by a TDE can also be produced by a cooling
neutron star.
In 2006, an HLX event called 3XMM J215022.4−055108
was detected by the XMM-Newton x-ray observatory.
The name’s a mouthful so I’m gonna call
it “J2150”. Anyway, it seemed to originate
in the vicinity of a galaxy approximately
800 million light-years distant. A team of
astronomers led by Dacheng Lin at the University
of New Hampshire discovered the event in archival
data from the XMM-Newton and Chandra X-Ray
observatories.
Now, a previous image of the galaxy made by
the Hubble Space Telescope in 2003 revealed
an optical counterpart to the source, which
appeared to be just slightly off-center from
the X-ray source.
Lin’s team made follow-up observations with
Swift in 2014 and Chandra in 2016. These observations
showed the X-ray flux had decreased at a rate
that was consistent with a tidal disruption
event involving a black hole weighing 50,000
to a hundred-thousand solar masses.
That’s an encouraging result, but the same
X-ray cooling curve could also be mimicked
by a neutron star in a low-mass binary system.
If its crust were heated in a large accretion
outburst, it would create an X-ray flare similar
to the J2150 event and cool off in the same
manner.
Still, Lin and his team had good reason to
believe this wasn’t due to a nearby neutron
star flare-up. For one thing, the apparent
source seemed to be right next to a large,
lenticular galaxy, dubbed "Gal 1". Several
smaller galaxies appear to surround it, including
a slightly larger dwarf dubbed “Gal 2”.
The two galaxies appear to be connected by
a stream of gas and stars that were stripped
from Gal 2 as it was tidally disrupted by
Gal 1.
But proximity in the sky to a galaxy isn’t
proof it can’t be a nearby neutron star.
Images of galaxies are photobombed by nearby
stars in our galaxy all the time.
If the source of the J2150 event were a star,
it would appear as a point source. But a small
galaxy or cluster would appear as an extended
object. Unfortunately, the 2003 Hubble observation
was designed to resolve the Gal 1 galaxy,
not the J2150 source. So there wasn’t any
way to determine if the source was a dwarf
galaxy, a globular cluster, or just a fuzzy
image of a foreground star.
To settle the question, the Hubble Space Telescope
observed the galaxy once again in 2018. The
difference from 2003 and 2018 may not seem
like much at first, but it was it was a huge
improvement, allowing the team to resolve
the source as a compact, but extended object.
J2150 was extragalactic in origin!
Accounting for its apparent brightness and
distance, Lin’s team determined the source
object is roughly 176 light-years in diameter
with a mass of about 10 million suns. It’s
likely the remnant nucleus of a tidally stripped
dwarf galaxy rather than a massive globular
cluster.
The new Hubble observations allowed the team
to refine their measurements of the X-ray
spectrum of the flare. They found it was caused
by a 50,000 solar-mass black hole shredding
a passing star.
This makes J2150 the strongest Intermediate
Black Hole candidate to date. We may have
finally have found the missing link the evolution
of supermassive black holes.
That in turn will help us to answer questions
like ‘what came first, the galaxy or the
black hole’? Did the black hole build the
galaxy? Did the galaxy build the black hole?
Or did they build each other?
Our best chance of answering these questions
will come when the James Webb Space Telescope
launches sometime in…well who knows now.
But even though some science has been delayed,
it hasn’t stopped. And neither should you.
The night sky still calls to us, which is
why I invite you to step out every now and
then and see what the Universe is doing.
In fact, it's super easy to navigate the sky
with Stellarium Mobile Plus. I’ve been using
Stellarium desktop to depict the night sky
here on Launch Pad Astronomy and in my classroom
for a number of years, so I was very happy
to learn that I can now take it with me when
I’m gazing at the night sky.
To that end, you can switch to a night-vision
mode, zoom in on planets, moons, galaxies,
and nebulae, track the planets and even satellites
as they pass overhead, all while experiencing
the most realistic depiction of the night
sky.
And if you have a motorized telescope, you
can control it with Stellarium’s built-in
Telescope Server Protocol. You don’t even
have to have an internet connection.
Stellarium Mobile Plus puts Stellarium’s
elegant combination of power and simplicity
into the palm of your hand. It’s available
now on Apple's App Store and on Google Play,
and I invite you to download it today. I’ll
have a link in the description below.
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