When the venerable Hubble Space Telescope conducted deep-field studies of the early universe, it discovered something that baffles astronomers to this day. When the universe was only a few billion years old, several large galaxies already existed. This mystery was further deepened by the deployment of the James Webb Space Telescope, which observed an abundance of bright, pre-existing galaxies. For astronomers, this raised the question of how such a huge and evolved galaxy could exist so soon after the Big Bang.

To address this mystery, a team at the Max Planck Institute for Radio Astronomy (MPIfR) Atacama Large Millimeter/Submillimeter Array (ALMA) Observe the center of SPT2349-56. This huge proto-galactic cluster is Atacama Pathfinder Experiment (APEX) Just 1.4 billion years after the Big Bang. There they observed four tightly interacting galaxies forming new stars at an astonishing rate. Their findings suggest that giant elliptical galaxies may have formed by the rapid collapse of early galaxy clusters.

The team behind this discovery was led by Nikolaus Sulzenauer, a postdoctoral researcher at MPIfR and the University of Bonn. He was joined by an international team from universities and research institutions around the world, including Herzberg Astronomy and Astrophysics, the Harvard-Smithsonian Center for Astrophysics, the Flatiron Institute Center for Computational Astrophysics, Enrico Fermi Institute, the European Southern Observatory (ESO), NASA’s Jet Propulsion Laboratory, and multiple universities. Their research, “Huge core of a galaxy cluster at redshift 4.3‘ was published in the diary nature.

Traditionally, astronomers expected to find only young stars and galaxies with high levels of star formation in the early universe. Instead, they observed many elliptical galaxies with old stellar populations and little new star formation. The predominant cosmological model suggests that there was not enough time for giant elliptical galaxies to form. Nikolaus Sulzenauer, postdoctoral researcher at MPIfR and the University of Bonn and lead author of the study, explained in an MPIfR press release:

In a universe where larger galaxies grow hierarchically through gravitational interactions and mergers of smaller components, some giant ellipses must have formed quite differently than previously thought. Instead of slowly building up mass over 14 billion years, giant elliptical galaxies could emerge rapidly within just a few hundred million years.

It could be formed through the collapse and merging of major primordial structures during the sun’s orbit around the center of the Milky Way galaxy. We found that the structures with the highest densities must have first separated from the expansion of the universe at just 10% of the current age of the universe, and then entire primordial galaxy clusters must have assembled rapidly.

SPT2349-56 holds the record for the highest rate of star formation in the early Universe, giving astronomers a rare glimpse into the earliest galaxy clusters. The researchers used ALMA to observe the cold gas and dust at the center of protoclusters, the material in which new stars form. There, they observed four closely interacting galaxies ejecting giant tidal waves of ionized gas clouds at speeds of about 300 km/s (186 miles per second), spreading over an area much larger than the Milky Way. At submillimeter waves, the brightness of these arms increased by a factor of 10 due to shock heating waves that excited ionized carbon atoms in the clouds.

*This radio image of protostar cluster SPT2349-56 shows the intensity of ionized carbon (CII) emitted at a wavelength of 158 micrometers. Credit & ©: N.Sulzenauer/MPIfR*

Surprisingly, they observed that a new star forms at a rate of one every 40 minutes, compared to the full year it takes for a small number of new stars to form in today’s Milky Way. Sulzenauer pointed out:

This bright emission allowed them to precisely measure the movement of gas within this gravity-ejected spiral, which resembles a bead of thread surrounding the core of a protostar cluster. Surprisingly, they found that the mass of tidal debris was connected to a chain of 20 more colliding galaxies in the outer part of the collapsing structure.

This suggests a common origin. For the first time, we are witnessing the beginnings of a cascading convergence transformation. Most of the 40 gas-rich galaxies in this core will be destroyed and eventually turn into giant elliptical galaxies within 300 million years, or in a fraction of a second.

The team was aided by detailed numerical simulations performed by two undergraduate students at the University of British Columbia. These are consistent with ALMA observations and previous studies of old galaxy clusters, indicating that large-scale mergers occurred simultaneously throughout the history of the universe. Their findings may also help explain how heavier elements such as carbon (one of the fundamental building blocks of organic chemistry and life) are heated and transported throughout early galaxy clusters. Scott Chapman, a researcher at Dalhousie University and lead author of the study, said:

While our findings provide exciting new insights into the rapid assembly of elliptical galaxies, the various interactions between merger shocks, gas heating due to supermassive black hole growth, and their influence on the fuel for star formation remain great mysteries. Although it may be too early to claim that we fully understand the “infancy” of giant ellipsoids, we have come a long way by linking tidal dust in primordial clusters to the formation process of the giant galaxies located in today’s clusters.

Read more: MPIfR