Ancient Galaxy Megamergers

The ALMA and APEX telescopes have peered deep into space — back to the time when the Universe was one tenth of its current age — and witnessed the beginnings of gargantuan cosmic pileups: the impending collisions of young, starburst galaxies. Astronomers thought that these events occurred around three billion years after the Big Bang, so they were surprised when the new observations revealed them happening when the Universe was only half that age! These ancient systems of galaxies are thought to be building the most massive structures in the known Universe: galaxy clusters.

Artist’s impression of ancient galaxy megamerger. Image credit: ESO (CC BY 4.0)

Two international teams of scientists led by Tim Miller from Dalhousie University in Canada and Yale University in the US and Iván Oteo from the University of Edinburgh, United Kingdom, have uncovered startlingly dense concentrations of galaxies that are poised to merge, forming the cores of what will eventually become colossal galaxy clusters.

Peering 90% of the way across the observable Universe, the Miller team observed a galaxy protocluster named SPT2349-56. The light from this object began travelling to us when the Universe was about a tenth of its current age.

The individual galaxies in this dense cosmic pileup are starburst galaxies and the concentration of vigorous star formation in such a compact region makes this by far the most active region ever observed in the young Universe. Thousands of stars are born there every year, compared to just one in our own Milky Way.

The Oteo team discovered a similar megamerger formed by ten dusty star-forming galaxies, nicknamed a “dusty red core” because of its very red colour.

Iván Oteo explains why these objects are unexpected: “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. So, finding numerous dusty starbursts shining at the same time like this is very puzzling, and something that we still need to understand.”

These forming galaxy clusters were first spotted as faint smudges of light. The two faint glows are not single objects, but are actually composed of fourteen and ten individual massive galaxies respectively, each within a radius comparable to the distance between the Milky Way and the neighbouring Magellanic Clouds. (ESO)

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