Why were some ancient galaxies so bright? Supercomputer probes mystery

Not all astronomy is about gazing at stars. By creating a galaxy inside a powerful supercomputer, scientists say they’ve developed a model that may explain how some of the brightest galaxies in the early universe came to be.

The findings, described in the journal Nature, could help solve a long-standing mystery about the origins of these luminous objects in the early cosmos.

The brightest denizens of the cosmos today literally pale in comparison to the behemoths that populated the early universe just 3 billion years after the Big Bang.

“During that epoch, known as cosmic noon, the average star-formation rate across the cosmos was 100 times higher than it is at present, and individual galaxies were growing commensurately rapidly,” Romeel Davé of the University of the Western Cape, who was not involved in the study, wrote in a commentary. “This was illustrated by the surprising discovery, more than a decade ago, of galaxies whose star-formation rates during that era were 1,000 times the Milky Way’s current output — no such galaxies are seen in the present-day universe.”

Even though these monstrous galaxies are bursting at the seams with bright stars, astronomers didn’t even know they existed until relatively recently because the visible light that’s being generated by the stars is actually absorbed by the massive amounts of dust surrounding the galaxy and re-emitted at longer, “redder” wavelengths — rendering them essentially invisible to optical telescopes.

“Usually, young stars are covered with a veil of dust, but in this case, the dust is covering the whole galaxy itself,” said study coauthor Dusan Keres, an astrophysicist at UC San Diego. “So it’s an unusual situation.”

But once astronomers built radio telescopes that could pick up longer wavelengths of light, these giants started popping up around the night sky.

Why did the early cosmos have such active, monstrous galaxies — called submillimeter galaxies, for the wavelengths of light at which they emit most strongly — which aren’t seen at all in the universe today?

Scientists have argued over two main theories. Perhaps these enormous galaxies were the result of violent galactic mergers, when two gas-rich galaxies crashed into each other. After all, that’s how many of the brightest galaxies in the universe today come to be (even though they’re admittedly much dimmer than their predecessors from more than 10 billion years ago).

On the other hand, perhaps these galaxies simply are long-lived galaxies that grew to their impressive size and brightness by steadily gathering more and more gas over a much longer period, perhaps a billion years or so.

Unfortunately, it’s hard to tell which one is true just by looking at the galaxies themselves, Keres said.

“They were observed with these radio telescopes that have relatively poor resolution, so we only see them as a smudge in the sky … so we couldn’t see much detail in there,” Keres said.

Instead, the researchers ran a sophisticated simulation of the dynamics of one such galaxy, using a powerful supercomputer at the Texas Advanced Computing Center. They took into account the emissions from the stars themselves, the obscuration by the dust and the re-radiation of the starlight in longer wavelengths.

“We have much more detailed modeling of the star formation and what the star formation is doing to the galaxy itself,” Keres said.

The model showed that, in fact, these galaxies really could grow steadily over long periods, without the need for a violent galactic smashup.

“Our model is showing that what’s powering submillimeter emission of these galaxies is constant bombardment by smaller galaxies and lots of gas that they are eating from their surroundings,” Keres said.

This was made much easier by the fact that the universe was much denser in the past than it is today, with more gas and dust squeezed into a smaller volume. A galaxy that thrived some 3 billion to 2 billion years after the Big Bang, for example, would have inhabited a cosmos that was roughly 20 to 50 times denser than it is today, Keres said. Because the universe is expanding, it gets sparser with every passing eon.

This doesn’t mean that a galaxy merger could not possibly create submillimeter galaxies — but they would probably be in the minority, Davé said.

“What is particularly encouraging is that the authors did not tune the simulations so as to reproduce [submillimeter galaxies]: rather, they simply used a state-of-the-art galaxy-formation model and ran it at the highest currently feasible numerical resolution — and a plausible [submillimeter galaxy] emerged,” Davé said.

Of course, this is just a simulation of a single galaxy, the scientists pointed out. More simulations would need to be done to show whether this could really be the rule among this population of bright, ancient galaxies.

And newer radio telescopes like ALMA (short for Atacama Large Millimeter/submillimeter Array) could finally shed clearer light on what might be happening in these distant objects.

“There are still a lot of unanswered questions,” Keres said.

In the meantime, Davé said, the authors of the study “have presented the first impressively viable model of [submillimeter galaxy] formation, allowing us a tantalizing glimpse behind the mask of these behemoths of deep space.”

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