Using the Euclid Space Telescope, scientists have discovered an incredible 1.5 trillion orphan stars floating through a massive cluster of thousands of galaxies, one of the largest structures in the universe.
These orphan stars, torn from their own galaxies, fill the space between the galaxies of the Perseus cluster with eerie blue light. This so-called “intracluster” light is so faint that it is many thousands of times darker than the night sky above Earth.
By observing this light within a cluster in the Perseus cluster, which is 240 million light-years from Earth and has a mass equivalent to about 650 trillion suns, Euclid could help scientists better understand where the faint light component of the galaxy cluster comes from and the origin of the cosmic orphans that emit it. .
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Euclid was launched from Cape Canaveral, Florida on a SpaceX Falcon 9 rocket on July 1, 2023. Euclid’s primary mission is to investigate dark energy, the mysterious force accelerating the expansion of the universe, and dark matter, “invisible” matter that does not interact with light and it is not composed of atoms like the “everyday” things that surround us.
However, despite being designed to peer into the invisible “dark universe”, the telescope was also able to detect light emanating from between galaxies in the Perseus galaxy cluster.
“We were surprised by our ability to see so far into the outer regions of the cluster and discern the subtle colors of this light,” team leader and University of Nottingham scientist Nina Hatch said in a statement. “This light can help us map dark matter if we understand where the stars within the cluster come from. By studying their colors, luminosities and configurations, we found that they originate from small galaxies.”
Orphan stars have the blues
The key to understanding the orphan stars in Perseus was Euclid’s ability to see the faintest light in the cluster, the light within the cluster, coming not from its galaxies but between them.
“This diffuse light is more than 100,000 times fainter than the darkest night sky on Earth,” said Matthias Kluge, a member of the team and the Max-Planck Institute for Extraterrestrial Physics. “But it’s distributed in such a large volume that when we add it all up, it accounts for about 20% of the brightness of the whole cluster.”
The orphan stars that Euclid saw in the Perseus cluster are distinguished by their characteristic blue color and loose clustering. These features allowed Hatch and colleagues to trace their origins.
The team found that some of these free-roaming stars in intracluster space were dragged away from the edges of galaxies through interactions with other galaxies. The other orphan stars they found came from smaller dwarf galaxies in the Perseus cluster that were completely disrupted.
What the team discovered next surprised them. After being torn from their parent galaxies, stars within a cluster are expected to begin orbiting the largest galaxies in the cluster in which they found themselves isolated, almost like a lost child at the mall gravitating toward the nearest adult.
Hatch and colleagues, however, did not find it in Euclid’s Perseus. Instead, they saw orphan stars orbiting a point between the cluster’s two brightest galaxies, NGC 1275 and NGC 1272.
“This new observation suggests that the massive Perseus cluster may have recently experienced a merger with another group of galaxies,” said team member and University of Nottingham astronomer Jesse Golden-Marx. “This recent merger could have caused a gravitational disturbance, causing the most massive galaxies or orphan stars to deviate from their expected orbits, leading to the observed misalignment.”
The same researchers also used Euclid’s sensitive visible-light capabilities to spot 50,000 free-flying densely packed and spherical collections of tens of thousands to millions of stars called “globular clusters” in the Perseus galaxy cluster. The scattered light within the cluster appears to be distributed in a similar fashion to the globular clusters in Perseus, so these conglomerates of stars appear to be the source of at least some of that light.
Stars in these globular clusters lack a high concentration of “metals,” the term astronomers use for elements heavier than hydrogen and helium. This implies to the team that the globular clusters in the Perseus galaxy cluster drifted inward from a vast collection of outer edge galaxies, which are also “metal-poor”.
Globular clusters are a dominant factor in dwarf galaxies, meaning that some of the light inside the cluster may come from the remnants of such small galaxies that have been torn apart by the tidal forces generated during encounters with more massive galaxies.
The team also discovered from Euclid’s observation of Perseus that the number of small dwarf galaxies in this galaxy cluster increases as one moves away from the center of the cluster.
The research helps confirm Euclid’s ability to understand the evolution of galaxies and galaxy clusters and, therefore, how the universe looked like it does to us today.
Excitingly, these findings are among the first science results from Euclid’s Early Release Observations, which represent just the first 24 hours of Euclid’s observations before it began observing its main science targets, billions of galaxies across more than a third of the sky on February 14, 2024.
The team’s research is featured on the paper repository website arXiv.