Ancient galaxy megamergers
ALMA OBSERVATORY/DICYT 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.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX), 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, by combining observations from ALMA and the APEX.
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, using the South Pole Telescope and the 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 fourteen and ten individual massive galaxies respectively, each within a radius comparable to the distance between the Milky Way and the neighbouring 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 VLT are also identifying additional galaxies,” comments Carlos De Breuck, ESO astronomer.
Current theoretical and computer models suggest that protoclusters as massive as these should have taken much longer to evolve. By using data from ALMA, with its superior resolution and sensitivity, as input to sophisticated computer simulations, the researchers are able to study cluster formation less than 1.5 billion years after the Big Bang.
“How this assembly of galaxies got so big so fast is a mystery. It wasn’t built up gradually over billions of years, as astronomers might expect. This discovery provides a great opportunity to study how massive galaxies came together to build enormous galaxy clusters,” says Tim Miller, a PhD candidate at Yale University and lead author of one of the papers.