Ancient Galaxy Megamergers
ALMA and APEX discover massive conglomerations of forming galaxies in the early Universe
Using the ALMA and APEX telescopes in Chile, two international research teams with participation from scientists of the Max Planck Institute for Radio Astronomy in Bonn, Germany, have uncovered startlingly dense concentrations of galaxies that are poised to merge, forming the cores of what will eventually become colossal galaxy clusters. The results are presented in two research papers to appear in the journals Nature and The Astrophysical Journal.
The Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) 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.
Two international teams 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, used both telescopes, ALMA and APEX, to uncover 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 first team observed a galaxy protocluster named SPT 2349-56. 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. As many as 15 000 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.
These forming galaxy clusters were first spotted as faint smudges of light, using the South Pole Telescope and the Herschel Space Observatory. Subsequent APEX and ALMA 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 glows spotted by APEX and Herschel 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.
“The duration of the starburst event in each of the galaxies is short compared to the evolution time scale of the proto-cluster”, explains Axel Weiß from the Max Planck Institute for Radio Astronomy who is co-author on both publications. “The fact that we see so many starburst galaxies in both clusters at the same time suggests either a so far unknown mechanism to trigger the activity over several hundred thousand light years or the presence of gas flows from the cosmic web to replenish the gas supply in the active galaxies.”
"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, astronomer at ESO, the European Southern Observatory.
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", concludes Tim Miller, a PhD candidate at Yale University and lead author of the paper in Nature.
The Atacama Pathfinder Experiment (APEX) is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), Onsala Space Observatory (OSO), and the European Southern Observatory (ESO) to construct and operate a modified prototype antenna of ALMA (Atacama Large Millimetre Array) as a single dish on the Chajnantor plateau at an altitude of 5,100 metres above sea level (Atacama Desert, Chile). The telescope was manufactured by VERTEX Antennentechnik in Duisburg, Germany. The operation of the telescope is entrusted to ESO.
The Atacama Large Millimeter/submillimeter Array (ALMA) is an international partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan, together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. ALMA -the largest astronomical project in existence- is a single telescope of revolutionary design, composed of 66 high precision antennas located on the Chajnantor plateau, 5000 meters altitude in northern Chile.
MPIfR affiliations: Axel Weiß is co-author in both publications and Maria Strandet, who just received her PhD in the IMPRS research school at MPIfR, is co-author in the Nature paper.