Massive primordial galaxies found in ‘halo’ of dark matter

Observations of two galaxies made with the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope suggest that large galaxies formed faster than scientists had previously thought.

The Aurora Australis, or Southern Lights, over the South Pole Telescope at NSF's Amundsen-Scott South Pole Station. Image credit: Dr. Keith Vanderlinde, NSF

The two galaxies, first discovered by the South Pole Telescope at NSF's Amundsen-Scott South Pole Station in Antarctica, were massive and star-filled at a time when the cosmos was less than a billion years old.

The observation came as a surprise, considering astronomers had thought that the first galaxies, which formed just a few hundred million years after the Big Bang, were similar to today's dwarf galaxies -- collections of stars much smaller than the Milky Way. After a few billion years, these early, smaller galaxies became the building blocks of the larger galaxies that came to dominate the universe, scientists believed.

But the latest ALMA observations push this epoch of massive-galaxy formation back further into the past, as the two galaxies were giants when the universe was only 780 million years old. ALMA also revealed that these large galaxies were nestled inside an even-more-massive cosmic structure, a halo of dark matter several trillion times more massive than the sun.

The discovery provides new details about the emergence of large galaxies and the role that dark matter plays in assembling the most massive structures in the universe.

Viewing distant galaxies means looking back through time, in a sense. The energy from those objects takes so long to reach Earth that researchers today view events that occurred billions of years ago.

The astronomy team captured data from these two galaxies as they were during a period of cosmic history known as the Epoch of Reionization, when most of intergalactic space was suffused with an obscuring fog of cold hydrogen gas. As more stars and galaxies formed, their energy eventually ionized the hydrogen between the galaxies, revealing the universe as we see it today.

The observations showed the two galaxies in such close proximity -- less than the distance from the Earth to the center of our galaxy -- that they were certainly on course to merge and form the largest galaxy ever observed in the Epoch of Reionization.

The galaxies that the team studied, collectively known as SPT0311-58, were originally identified as a single luminous source. (National Science Foundation)

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