Free Astronomy Magazine July-August 2018

21 SPACE CHRONICLES ous 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 observa- tions 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 t w o faint glows are not single objects, but are actually c ompo s ed of fourteen and ten individ- ual massive galax- ies respectively, each within a radius compara- ble to the distance between the Milky Way and the neighbour- ing 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 in- strument on ESO’s VLT are also identifying addi- tional 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 for- mation 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 candi- date at Yale University and lead author of one of the papers. ! T his artist's impression of SPT2349-56 shows a group of interacting and merging galaxies in the early Universe. Such merg- ers have been spotted using the ALMA and APEX telescopes and represent the formation of galaxies clusters, the most massive objects in the modern Uni- verse. Astronomers thought that these events occurred around three bil- lion years after the Big Bang, so they were sur- prised when the new ob- servations revealed them happening when the Uni- verse was only half that age! [ESO/M. Kornmesser] JULY-AUGUST 2018