Astronomers Found Two Massive Galaxies Amid an Ocean of Dark Matter

A high rate of star formation within the galaxies suggests the two are merging, researchers said.

Astronomers have found a pair of surprisingly gigantic galaxies from the early universe, and one is the most massive galaxy ever seen from when the universe was young. Observations show the pair of big galaxies existed less than 800 million years after the Big Bang, and their size doesn’t conform to current cosmological models, which say early galaxies were much smaller than today’s galaxies.

Additionally, the discovery provides new details on the role that dark matter played in assembling large structures in the universe. Astronomers say the amount of dark matter that “clumped” in one area in order to create galaxies this large would have been quite rare.

“The fact that we see two big galaxies together strengthens the case that we’re looking at a particularly dense region in the universe, where gravity has assembled a large mass of dark matter in which to form these galaxies,” Dan Marrone from the University of Arizona in Tucson told Seeker.

“However,” he added, “the fact that one very massive dark matter halo has been found at this time is not too surprising, as there should be a handful in the universe according to current cosmology. If we saw a lot of these galaxies as we continued with our survey, it would be surprising and it might suggest that we don’t understand something about how dark matter structures forms.”

Marrone is the lead author on a new paper published in the journal Nature describing the findings.

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The two galaxies are quite close, separated by a distance less than that between Earth and the center of our galaxy. Astronomers expect that the pair will likely merge to form the largest galaxy that has ever been observed during the first billion or so years of cosmic history.

The pair of galaxies was originally spotted by the South Pole Telescope in Antarctica, and observations indicated that the duo might be one incredibly large light source. Complicating matters was that another galaxy was in the way, creating a gravitational lens, bending the path of the incoming light and magnifying the source. Scientists didn't know if the light source — known as SPT0311-58 — was heavily magnified and tiny or lightly magnified and massive. Normally, gravitational lensing can help increase spatial resolution of the background galaxy, but Marrone said in this case it was more of a nuisance because the lensing was quite weak.

That prompted observations with Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, which has better resolution. The team determined the light source was from two separate but close galaxies, and the light from these galaxies took around 13 billion years to reach Earth, so they appear as they did when the universe was young.

Deconstructing the lensing process provided intriguing details about the galaxies, such as a surprisingly large amount of gas and dust for galaxies during that period in the universe. Also, the galaxies were forming stars at an extremely high rate, about 2,900 solar masses per year. Astronomers think this means that the two galaxies are merging.

“Galaxies often interact and merge to form larger galaxies, and these interactions can cause the galaxies to form stars at a faster rate than they would otherwise,” co-author Chris Hayward said in an email to Seeker. Hayward is an associate research scientist at the Center for Computational Astrophysics at the Flatiron Institute in New York City.

The gravitational tidal forces exerted by the galaxies on one another can cause the gas in the galaxies to become denser. This makes the interacting galaxies especially bright and therefore easier to detect.

“We often find that the very brightest galaxies appear to be collisions of two galaxies near the end of the merger process, when the star formation enhancement is strongest,” Hayward explained. “The high star-formation rate inferred for this pair of galaxies may be a product of the interaction between the two galaxies.”

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Marrone added that early in the universe, galaxy collisions happened frequently, especially in regions with the strongest gravitational pull from the dark matter structure.

The prevailing theory was that smaller galaxies in the early universe were the building blocks to create larger galaxies, and that larger galaxies came into existence only after the first few billion years.

However, the team’s ongoing observations with ALMA have found other surprising examples of massive, star-filled galaxies seen when the cosmos was less than a billion years old. This suggests that smaller galactic building blocks were able to assemble into large galaxies quite quickly, and the new findings push back the time of massive-galaxy formation to when the universe was only 780 million years old.

“Mounting observational evidence with ALMA,” Marrone said in a statement, “has helped to reshape that story and continues to push back the time at which truly massive galaxies first emerged in the universe.”

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