Name: That Galaxy With No Dark Matter? It's Probably Not Real | SciShow News
Uploaded: 2020-04-15T16:55:46.000Z
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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

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

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

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

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

and it's only a quarter of the galaxy's total mass.

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,

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

Some of these things gobble up the dust and gas around them, generating a super hot accretion

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

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.

We have been able to figure out that the region around the black hole is crowded with dust

and roving stars. We've also determined that, theoretically, there should be both

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

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

the way from the center of the galaxy without being scattered.

There, nestled in a ring about a hundredth of a light-year from the supermassive black

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,

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.

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That Galaxy With No Dark Matter? It's Probably Not Real | SciShow News

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