A group of galaxies in the very early universe were unintentionally found by an astronomy master’s student while he was developing and testing astronomical software on existing data. The discovery not only shows the potential of the software but also sheds light on how massive structures are put together and why some galaxies stop forming stars.
The majority of galaxies do not enjoy exploring the cosmos by themselves. Instead, they typically congregate in enormous clusters or smaller groups that are contained within the densest concentrations of dark matter. Key insights into the evolution of galaxies as well as the distribution of dark matter and the geometry of our universe can be gained from the discovery and study of clusters and the galaxies that make up their members.
A fortuitous discovery
Nikolaj Sillassen, a master’s student at DTU Space and member of the Cosmic Dawn Center (DAWN), a collaboration between the Niels Bohr Institute and DTU Space, discovered a remarkable group of galaxies while developing a new software for automatic detection of distant galaxy clusters.
The group has been dubbed “HPC1001,” and it consists of ten galaxy members seen 12 billion years ago, when the universe was only 1.7 billion years old.
ShuowenJin, a Marie Curie Fellow at DAWN, is one of Nikolaj Sillassen’s collaborators. He claims, “Galaxy groups this early in the Universe’s history are extremely rare. So far, only a few of them have been discovered. Our discovery is intriguing not only because HPC1001 is one of the most distant groups, but also because it is the most compact assembly of galaxies discovered in our Universe to date.”
Galaxies that are dying
While some galaxies are actively star-forming, others abruptly stop producing new stars. One of the primary objectives of the Cosmic Dawn Center is to investigate the causes and evolution of this phase.
In this context, HPC1001 also proved to be a treasure trove: “Remarkably, the most massive galaxy member in this structure is ‘dying’—its star-formation activity is declining,” says Georgios Magdis, an associate professor at DAWN and study participant.
“This is an important indicator for massive structure evolution, and if confirmed, HPC1001 would be the earliest discovered structure in the maturing phase.”
The distance and “lookback time” to HPC1001 are calculated using an unreliable technique based on the observed colours of the individual galaxies. The astronomers will follow up their detection with more precise spectroscopic observations to confirm their measurements.
Future observations with the Atacama Large Millimeter Array in Chile and the Northern Extended Millimeter Array in France, among other astronomical facilities, will reveal the full nature of this structure, including whether HPC1001 will remain a small group or evolve into a massive cluster of up to 1000 galaxies.
The galaxy cluster was discovered during a synthesis project at DTU Space led by ShuowenJin and Georgios Magdis. The findings have been accepted for publication in Astronomy & Astrophysics.