

Astronomers Spot Two Massive Galaxies Surrounded by a Halo of Dark Matter at the Dawn of Time

Less than a billion years after the Big Bang, two titans speed toward each other.


Just 780 million years after the universe formed in the Big Bang, two galaxies speed to confront each other in a head-on collision that will lead to a merger between the two—and one of them is towing along a clump of dark matter larger than any spotted before.

The research paper, published today in Nature, highlights a little-understood era of the universe known as the Epoch of Reionization. This is when the first galaxies came together and lit up the universe by converting hydrogen from a neutral atom to an ionized state, making the universe more transparent.

Most galaxies of the era were believed to be small, low-mass dwarf galaxies. But the results of this study—and an unrelated paper also published in Nature today that highlights a supermassive black hole from this period, the oldest ever discovered—paint a different picture of the early universe. One of the galaxies in the pair known as SPT0311-58 is only slightly less massive than the Milky Way, though the other is much smaller.

The Milky Way's mass is equal to some 480 billion suns, while SPT0311-58 has about 440 billion solar masses. The smaller galaxy in the ancient pair is about 35 billion solar masses. A halo of dark matter surrounding the two is about 100 billion solar masses.

The galaxies, created via a composite image of several telescopes.
ALMA (ESO/NAOJ/NRAO), Marrone, et al.; B. Saxton (NRAO/AUI/NSF); NASA/ESA Hubble

Though the dark matter halo can’t be seen, its presence is inferred through gravitational interactions with the galaxies, which suggest it is enveloping both as they merge. The galaxies are messy in appearance as they haven’t had time to settle into a spiral (or other) shape due to their relative youth.

According to a NRAO press release, there are more galaxies waiting to be discovered in the same field. The pair was discovered by the South Pole Telescope, which is specifically attuned to the early universe, and follow-up observations were made with the Atacama Large Millimeter/submillimeter Array (ALMA).

“Our hope is to find more objects like this, possibly even more distant ones, to better understand this population of extreme dusty galaxies and especially their relation to the bulk population of galaxies at this epoch,” said Joaquin Vieira of the University of Illinois at Urbana-Champaign in a press release.

How the first galaxies formed in the ether of the early universe is one of the biggest questions in astronomy. Studying galaxies like SPT0311-58 could help scientists understand the strange dynamics of this ancient time in the cosmos.

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