Peering deep into space — an astounding 90% of the way across the observable Universe — two groups of astronomers led by University of Edinburgh’s Dr. Iván Oteo and Tim Miller from Dalhousie and Yale Universities have witnessed the beginnings of gargantuan cosmic pileups: the impending collisions of young, star-forming galaxies.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), Miller and co-authors have uncovered a very dense concentration of 14 galaxies that are poised to merge, forming the core of what will eventually become a colossal galaxy cluster.

This tightly bound galactic smashup — a galaxy protocluster named SPT2349-56 — is located approximately 12.4 billion light-years away, meaning its light started traveling to us when the Universe was only 1.4 billion years old, or about a tenth of its present age.

SPT2349-56’s individual galaxies are forming stars as much as 1,000 times faster than our Milky Way Galaxy and are crammed inside a region of space only about three times the size of the Milky Way.

The resulting galaxy cluster will eventually rival some of the most massive clusters we see in the Universe today.

“How this assembly of galaxies got so big so fast is a bit of a mystery, it wasn’t built up gradually over billions of years, as astronomers might expect,” said Miller, lead author of a paper published in the journal Nature (arXiv.org preprint).

“This discovery provides an incredible opportunity to study how galaxy clusters and their massive galaxies came together in these extreme environments.”

“Having caught a massive galaxy cluster in throes of formation is spectacular in and of itself,” added team member Dr. Scott Chapman, an astrophysicist at Dalhousie University.

“But, the fact that this is happening so early in the history of the Universe poses a formidable challenge to our present-day understanding of the way structures form in the Universe.”

Dr. Oteo and colleagues discovered a similar megamerger formed by 10 dusty star-forming galaxies, nicknamed a ‘dusty red core’ because of its very red color, by combining observations from ALMA and the Atacama Pathfinder Experiment (APEX) telescope.

“The lifetime of dusty starbursts is thought to be relatively short, because they consume their gas at an extraordinary rate. At any time, in any corner of the Universe, these galaxies are usually in the minority,” said Dr. Oteo, lead author of a paper published in the Astrophysical Journal (arXiv.org preprint).

“So, finding numerous dusty starbursts shining at the same time like this is very puzzling, and something that we still need to understand.”

SPT2349-56 and ‘dusty red core’ protoclusters were first spotted as faint smudges of light, using the South Pole Telescope and ESA’s Herschel Space Observatory. Subsequent ALMA and APEX observations showed that they had unusual structure and confirmed that their light originated much earlier than expected — only 1.5 billion years after the Big Bang.

The new high-resolution ALMA observations finally revealed that the two faint glows are not single objects, but are actually composed of 14 and 10 individual massive galaxies respectively, each within a radius comparable to the distance between the Milky Way and the neighboring Magellanic Clouds.

“These discoveries by ALMA are only the tip of the iceberg. Additional observations with the APEX telescope show that the real number of star-forming galaxies is likely even three times higher. Ongoing observations with the MUSE instrument on ESO’s Very Large Telescope are also identifying additional galaxies,” said ESO astronomer Dr. Carlos De Breuck, a member of Miller’s team.

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T.B. Miller et al. 2018. A massive core for a cluster of galaxies at a redshift of 4.3. Nature 556: 469-472; doi: 10.1038/s41586-018-0025-2

I. Oteo et al. 2018. An Extreme Protocluster of Luminous Dusty Starbursts in the Early Universe. ApJ 856, 72; doi: 10.3847/1538-4357/aaa1f1