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  • This episode of SciShow Space News is sponsored by Audible.

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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 outthat 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 anyto 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 Celsiuswhich 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 galaxyand 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

  • to more of them,

  • you can download an audiobook from Audible.

  • Many years ago I readHyperionby Dan Simmons in physical form, and I recently decided

  • to download the audiobook

  • And it's just wonderful to be taken back into that very bizarre and beautiful universe

  • It may be the best story that's ever happened….

  • ...and Audible also has plenty of stories that aren't science fiction, and they also

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  • To learn more, you can visit audible.com/scishowspace or textscishowspaceto 500 500.

  • So Audible knows that we sent you.

  • {♫Outro♫}

This episode of SciShow Space News is sponsored by Audible.

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