This episode is sponsored by Surfshark.

Get Surfshark VPN at surfshark.deals/SciShowSpace  and enter the promo code SciShowSpace for an 83% discount  and three extra months for free. [♪ INTRO]. The universe has been around  for nearly 14 billion years, and stars have been forming just about that  whole time, but not always at the same rate.

Like, more than 10 billion years ago,  there was a ton of gas to go around, so the universe was experiencing  peak star formation. But in the middle of that  period, something weird happened: a collection of galaxies just  shut off and stopped making stars. And it’s been a big mystery why.

But last week in the journal Nature, astronomers announced they may  have found the start of an answer. There’s a common astronomy metaphor out there that telescopes are time machines. The idea is that since light  travels at a finite speed, it takes time for the light from  a faraway object to reach us.

So, when we see a galaxy a  billion light-years away, we’re actually seeing it as  it was a billion years ago. And that can be super convenient. Like, this means that by studying galaxies  that are farther and farther away, astronomers can look back  in time and piece together how these structures have evolved.

For instance, we’ve learned that  some of the most massive galaxies, like ones with 100 billion stars,  formed fast, right after the Big Bang. But then, about half suddenly  stopped making new stars. And it’s not like they ran out of matter.

They were collecting all kinds of stuff  by gobbling up smaller, nearby galaxies. So, why wasn’t that stuff feeding star formation? Well, part of the answer might be that  galaxies need cold gas to make stars, since hot gas molecules are zooming  around too fast to clump together.

So, was something stopping cold  gas from turning into stars, or had the cold gas supplies run out? To try and learn more, the team behind  this new paper studied six of these big, dead galaxies, using two telescopes. They used the Hubble Space Telescope  to collect ultraviolet, visible, and near-infrared light, which  let them pinpoint the locations of the stars that had formed before the shutdown.

They also used the ALMA array in Chile,  which captures radio wavelengths. ALMA was used to measure the abundance  of cold dust, which astronomers then used to infer the amount of cold hydrogen  gas a galaxy would use to make more stars. In the end, the team found that these  galaxies were nearly devoid of cold dust, and by extension, of cold gas.

Literally, their gas tanks  appeared to have been emptied. And although this is helpful,  it doesn’t explain why. It’s possible all that gas  is still there, but maybe a supermassive black hole heated it  up, so it has to spend a lot of time cooling down before it can condense into a star.

Or maybe these galaxies quickly burned through what cold gas they had, and ejected the rest. Either way, we know that the rest of the  early universe was full of cold hydrogen, so something had to have happened to prevent these galaxies from pulling in more fuel. The good news is, that also means  there’s a clear line of questioning this team can take into future research.

Our next story comes from even further  back in the universe’s history. Last week, also in the journal Nature,  astronomers announced the discovery of some baby galaxies hiding amongst a  bunch of dust, something that hints that there are way more galaxies  out there than previously thought. About 380 thousand years after  the Big Bang, the universe was filled with a fog of hydrogen and helium gas.

But then, about 360 million years  later, the first stars came to life. They blasted radiation into  space, and it turned the neutral gas atoms in the fog  into electrically-charged plasma. So, bubbles of plasma began forming  and growing around the first stars, and the Epoch of Reionization  was officially underway.

This period lasted until the universe  was about a billion years old when enough energy had been  pumped out by the first stars and galaxies that the fog was effectively gone. And with less fog, it’s a lot easier to  study galaxies, so maybe unsurprisingly, most of the ones scientists have looked  at formed after the Epoch of Reionization. But of course, astronomers are still trying  to study those first stars and galaxies, ones that still look like they’re in that fog, since their light has taken  billions of years to reach us.

Most research into these galaxies has  involved looking for ultraviolet light, since that was the main kind of  light creating all that ionized gas. But UV light can also be absorbed by dust. So, in this new study, one team  decided to use our old friend ALMA to look for longer wavelengths of light.

They focused on two baby galaxies,  but noticed that there were strong signals at those longer wavelengths  that didn’t align with their targets. After a thorough analysis to identify  where these signals were coming from, the team concluded that these  were undiscovered/galaxies from the Reionization period, ones so  obscured by dust that any UV or visible light they’re emitting is getting completely blocked. One of them is even the most distant,  dust-obscured galaxy to date.

The light we’re seeing from it  was emitted 13 billion years ago. And there might be more like this. With all the dust astronomers have found in space, this team estimated that as  much as 20% of galaxies from the Epoch of Reionization are  out there hiding in the clouds.

In addition to follow-up studies from  ALMA, finding more of these hidden galaxies could be a great job for the  powerful James Webb Space Telescope, which is scheduled to launch  by the end of this year. Because if there’s one takeaway  from both papers this week, it’s that even when you learn one  thing, there’s always more to discover. But while you wait for the telescope to launch you should stream whatever you want, regardless of location restrictions, with  the help of today’s sponsor, Surfshark.

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