Massive galactic clusters were thought to have formed 3 billion years after the big bang when their component galaxies started merging. However, recent observations of starburst galaxies now reveal that they began merging and forming galactic clusters much sooner – 1.5 billion years sooner to be exact.

Galactic clusters contain hundreds to thousands of galaxies and are thus one of the most massive structures in the Universe, with super clusters being the most massive. These galactic clusters were previously thought to have formed 3 billion years after the big bang when young star-forming galaxies known as starburst galaxies merged to form a proto-cluster, that is a group of galaxies that are beginning to form a cluster. Starburst galaxies have an exceptionally high rate of star formation, about 1000 times greater than that of a normal galaxy such as the Milky way which has a star formation rate (SFR) of approximately 1 solar mass per year. Due to their high SFR, starburst galaxies exist for relatively short timescales and are thus considered to be just a short phase in the galactic lifetime.

In an effort to study these high SFR galaxies, two teams led by Oteo and Miller, respectively, both used the South Pole Telescope and the Herschel Space Observatory to discover two proto-clusters with redshifts compatible with the early universe.

Further observations with the Atacama Large Millimeter Array (ALMA) and Atacama Pathfinder Experiment (APEX) resolved the intrinsically luminous sources as galactic proto-clusters. Oteo’s team identified an extreme proto-cluster of galaxies with a core formed by at least 10 dusty star-forming galaxies distributed over a region of space 6 million trillion miles wide and with a SFR of 6500 solar masses per year. Similarly, Miller’s team identified the source SPT2349-56 as a galactic proto-cluster containing 14 gas rich galaxies distributed in a region of 2.5 million trillion miles in diameter. Subsequent spectroscopic analysis of both sources revealed a redshift of z = 4.002 and z = 4.31 which is 1.6 billion years and 1.4 billion years after the big bang, respectively.

Not only are both these proto-galaxies the most active and dense regions of star formation in the Universe, they are also much older than expected – twice as old as expected!

Equally puzzling is the finding that the gas depletion rates are very high, depleting on timescales much lower than that of the ongoing star formation – leading the authors to suggest the presence of gas flow from the cosmic web sufficient to sustain star formation over much longer times than the estimated gas depletion times.

These findings open up the question as to how these galaxies got so big so quickly and how star formation was sustained. Future observations and analysis will help to confirm these findings and reveal possible new theoretical models which may lead to a new understanding of cosmological structure formation and evolution. One thing for sure is – the current models don’t agree with observations.

Amira Val Baker