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{♫Intro♫}
A little over a year ago, we covered a mindblowing
galaxy on SciShow Space News.
It’s commonly called NGC 1052-DF2, or DF2
for short, and scientists
thought it contained virtually no dark matter.
This is a hypothetical kind of matter that’s
invisible and that doesn’t interact
with regular matter, but scientists are pretty
confident it exists.
This finding was bizarre, because as far as
we know,
you need a good amount of dark matter to keep
stars moving the way they do.
So this discovery was raising all kinds of
questions about how galaxies like this
could form and stay together.
Some researchers even suggested that, if this
was confirmed,
it would be one of the biggest astronomy findings
in years.
Except, as it turns out… that discovery
was probably wrong.
According to a paper published in this month’s
Monthly Notices of the Royal Astronomical Society,
DF2 is a perfectly ordinary galaxy with a
normal amount of dark matter.
All that confusion was probably just caused
by some fuzzy measurements.
In the original study, the authors estimated
the mass of DF2’s stars based on the
galaxy’s brightness and distance. Then,
they measured how other galaxies were moving
around it.
Normally, that motion can’t only be explained
by the mass of the stars,
so you need to add in a certain amount of
dark matter to balance things out.
But that wasn’t true in this case. All those
numbers suggested that you needed very little
dark matter —
if any — to explain the movement of those
other galaxies.
Unfortunately, though, that distance measurement
might not have been all that accurate.
And If you go back to the original study,
this isn’t actually that surprising.
In the paper, none of the data seemed to agree
on a single figure for the distance,
and the authors of the new study point out
that some of the methods were used
in a range where they hadn’t been previously
tested.
In studies like this, that isn’t always
a sign that like everything is wrong,
since calculating distance in the giant emptiness
of space is hard.
It just means that sometimes you need more
data.
And in this case, that new data led to new
results.
In this month’s study, an international
team doubled down on the distance measurements,
examining five datasets to try to weed out
any ambiguity.
They looked at the relationship between the
colors and brightnesses of DF2’s stars,
examined similarities with other galaxies
and objects nearby, and even compared
how much the stars flickered when they were
counting them —
all of which are closely linked to distance.
After all of this, they finally converged
on a distance that every dataset seemed to
agree on:
42 million light-years, instead of about 65
million.
That number had huge implications for estimating
DF2’s mass.
With the new data, the galaxy’s stellar
mass works out to be less than half of what
we previously thought,
and it’s only a quarter of the galaxy’s
total mass.
The rest, then, must be dark matter.
This brings DF2 in line with what we consider
normal in the universe:
galaxies of visible stars held together with
a massive, invisible shroud of dark matter.
So at least this time, it’s back to business
as usual!
In other news, scientists have cleared up
another mystery much closer to home —
a mystery about the supermassive black hole
at the center of the Milky Way.
It’s called Sagittarius A star. It’s about
4 million times the mass of the Sun,
and it’s 26,000 light-years away.
The weird thing about this object is actually
how little we know about it.
We know quite a lot about black holes in general,
but we have surprisingly
little insight into the one in our own galaxy.
For instance, we don’t know how it accumulates
material,
or how the disk of stuff around it, known
as the accretion disk, actually works.
But in a report published last week in the
journal Nature, a team from the U.S.
has managed to take the first picture of the
cool gas in the accretion disk.
This confirms something we’ve believed for
years, and the team hopes their discovery
will shed new light on how material falls
into black holes.
To understand this study, it helps to know
that Sagittarius A star is pretty shy, as
far as black holes go.
Some of these things gobble up the dust and
gas around them, generating a super hot accretion
disk
that shines really brightly if you look at
it with a radio telescope.
That’s actually how we got the incredible
photo of the black hole in another galaxy
that was published earlier this year.
But Sagittarius A star is a little more chill.
It’s picking gently at its accretion disk
and lets out very little in the way of radiation.
And little radiation means little information.
That’s why we don’t know a lot about it.
We have been able to figure out that the region
around the black hole is crowded with dust
clouds
and roving stars. We’ve also determined
that, theoretically, there should be both
hot and cold gases
that form a spinning accretion disk around
it, too.
But until recently, we had only been able
to see the hottest gas.
It’s a scorching 10 million degrees Celsius,
so it glows brightly in X-ray light.
But it also doesn’t behave like we’d expect.
Instead of rotating, this gas just falls in
towards the monster black hole.
So in this new study, researchers set out
to find the cooler gas,
hoping to find a more structured accretion
disk.
Using a radio telescope called ALMA, they
focused on the specific wavelength of radiation
that’s emitted
as hydrogen atoms form from colliding electrons
and protons.
This radiation is a hallmark of cooler conditions,
and its signal is strong enough to make it
all
the way from the center of the galaxy without
being scattered.
In the end, this work paid off.
There, nestled in a ring about a hundredth
of a light-year from the supermassive black
hole,
was the elusive cool gas scientists had hoped
to find.
It’s only 10,000 degrees Celsius — which
isn’t exactly cold by human standards,
but is much cooler than the other stuff.
This study didn’t only find the gas, though.
The researchers were also able to measure
the gas’s relative movement, using what’s
known as redshift.
This is the phenomenon where objects moving
away from us look more red,
since their light waves get stretched out,
and objects moving toward us look more blue,
since their light gets compressed.
It’s kind of like what happens with sound
when an ambulance drives past you blaring
its sirens.
When examining the cool gas around Sagittarius
A star, the team found that on one side
the gas was redshifted, and on the other it
was blueshifted.
This, they said, was clear evidence that the
gas was rotating, and this image was the first
to show that.
So for the first time, we’ve been able to
take a picture of the spinning accretion disk
around the black hole in our own galaxy — and
confirm that yes, it definitely exists.
This is a breakthrough for understanding accretion
on Sagittarius A star,
as well as its complex interactions with nearby
stars and nebulas.
If the DF2 study proved anything, it’s that
other scientists should validate these results.
But one way or another, it’s been an exciting
week in astronomy.
If all this talk of black holes and faraway
galaxies has made you start thinking about
science fiction,
I get it. Welcome to the club.
Stories are a great way to think differently
about the universe, and if you want to listen
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Many years ago I read “Hyperion” by Dan
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And it’s just wonderful to be taken back
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It may be the best story that’s ever happened….
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To learn more, you can visit audible.com/scishowspace
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So Audible knows that we sent you.
{♫Outro♫}
