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Submillimeter galaxies are ancient, dense, massive galaxies with up to 10 times the number of stars in the Milky Way, and for a long time, scientists couldn’t even figure out how they existed in the first place.

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For years, scientists have been studying a group of galaxies that shatters records. They're classified as elliptical galaxies, since they're elliptical in shape and don't have any recognizable structure. But unlike the billions of other ellipticals out there, these things are ancient, dense, and more massive than any group of galaxies we've ever seen.

They contain up to ten times the number of stars in the Milky Way. And not only are they record-holders, but for a long time, scientists couldn't even figure out how they existed in the first place. They didn't fit into the usual models for how ellipticals form.

And it wasn't until researchers uncovered a clue at the edge of the universe that they could finally tell the story of how the most massive galaxies came to be. Normally, elliptical galaxies form when spiral-shaped galaxies like the Milky Way collide and merge together. But studies of some of the most massive ellipticals show that about half of them had already finished forming by three billion years after the Big Bang.

And most spiral galaxies didn't exist until later. So it's pretty clear that these things didn't come about through the usual mergers. That wasn't the only strange thing about them, though.

The stars in these ancient neighborhoods are also packed incredibly tightly: between ten and a hundred times denser than in other ellipticals. And at first, astronomers couldn't figure out why that would happen, either. This mystery finally began to unravel in the 1990s, when scientists first figured out how to study the universe with submillimeter radiation.

Submillimeter wavelengths are longer than visible and infrared waves, but shorter than radio waves. They're often really tricky to observe from Earth because our atmosphere absorbs most of them. But in the late ‘90s, we finally developed the tools to study them.

In 1998, astronomers used a new camera on the James Clerk Maxwell Telescope in Hawai'i to create the first highly detailed picture of our universe at submillimeter wavelengths. And it revealed new, incredibly distant galaxies shining brightly with submillimeter radiation. They were at the very edge of the observable universe, and were some of the oldest galaxies we'd ever seen.

No one had discovered them before because these things were almost completely dark at the visible wavelengths. They released a ton of submillimeter radiation, though, because they're incredibly dusty. The visible light coming off their stars gets absorbed by the surrounding dust, and that heats up and re-emits the light at longer wavelengths, mostly in the submillimeter range.

These objects came to be called, creatively, submillimeter galaxies. Scientists think they formed when galaxies in the early universe grew massive enough to pull in huge amounts of gas and dust from their surroundings. This discovery turned out to be huge, not just because it was a new type of galaxy, but because it eventually helped us crack the secret of those ridiculously massive ellipticals.

See, aside from making them invisible, all of that dust also makes these submillimeter galaxies really active star-forming regions. New stars are being born there thousands of times faster than in the Milky Way, which puts out a measly one or two stars a year. And with so much star formation happening, scientists predicted that these galaxies would quickly burn themselves out.

In the end, they'd become compact elliptical galaxies with densely packed stars. If you think that's starting to sound familiar, you're not the only one. Apart from their density, further studies revealed that these submillimeter galaxies are eerily similar to today's most massive elliptical galaxies.

Like, a 2014 study even found that their stars have many of the same characteristics. That suggests that these dusty neighborhoods are precursors to the massive elliptical ones. And from there, the rest of the story pretty quickly fell into place.

Further modeling showed that the stars in submillimeter galaxies formed super fast, with star formation pretty much done in forty million years; a cosmic blink of the eye. Over the next few billion years, these compact elliptical galaxies wandered the universe, tugged here and there by the faint pull of gravity from other galaxies. Now and then, close enough encounters led to collisions.

Galaxies merged, mixing together their stars and creating much larger, but still incredibly dense, elliptical galaxies. And with that, the universe's giant elliptical galaxies were born, with dense cores made from stars several billion years older than the galaxy itself. This appears to finally solve the mystery of where the universe's most massive galaxies came from, and it's a huge step toward understanding the evolution of structure in our universe.

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