Using the Euclid Space Telescope, scientists have discovered a whopping 1.5 trillion orphan stars adrift in a massive cluster of thousands of galaxies, one of the largest structures in the universe.
These orphaned stars, torn from their own galaxies, fill the space between the galaxies of the Perseus cluster with ghostly 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 inside 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 can help scientists better understand where the faint light component of galaxy clusters comes from and the origin of the cosmic orphans that emit it .
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Euclid launched from Cape Canaveral, Florida atop a SpaceX Falcon 9 rocket on July 1, 2023. Euclid’s primary mission is to study dark energy, the mysterious force driving the expansion of the universe, and dark matter, the “invisible” matter that interacts with light and is not made up of atoms like the “everyday” things that surround us.
Despite being designed to peer into the invisible “dark space,” the telescope was also able to detect light emanating from 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 inside the cluster came from. By studying their colors, luminosities and configurations, we found that they came 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 inside the cluster that comes 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 team member and Max-Planck Institute for Extraterrestrial Physics Matthias Kluge. “But it is spread over such a large volume that when we add it all up, it makes up about 20% of the luminosity of the entire 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 the intracluster space were dragged away from the edges of galaxies through interactions with other galaxies. Other orphan stars they found came from smaller dwarf galaxies in the Perseus cluster that had been completely disrupted.
What the team discovered next surprised them. After being torn from their home galaxies, the stars within the cluster are expected to begin orbiting the largest galaxies in the cluster in which they find themselves isolated, almost like a lost child in the mall gravitating toward the nearest adult.
However, Hatch and colleagues did not find this with Perseus and Euclid. 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 undergone a merger with another group of galaxies,” said team member and astronomer Jesse Golden-Marx of the University of Nottingham. “This recent merger could have produced a gravitational perturbation that would have caused the most massive galaxies or orphan stars to deviate from their expected orbits, resulting in the observed misalignment.”
The same researchers also used the sensitive visible light capability of Euclid to observe 50,000 free-flying densely packed and globular collections of tens of thousands to millions of stars called “globular clusters” in the Perseus galaxy cluster. Diffuse light within the cluster appears to be distributed in a similar fashion to the globular clusters in Perseus, so these conglomerations of stars appear to be the source of at least some of this light.
The stars in these globular clusters lack a high concentration of “metals,” a term astronomers use for elements heavier than hydrogen and helium. This suggests to the team that the globular clusters in the Perseus galaxy cluster came in from a large collection of outer edge galaxies that are also “metal-poor”.
Globular clusters are the 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 found, based on Euclid’s observations 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 validate Euclid’s ability to understand the evolution of galaxies and galaxy clusters, and thus how the universe looks like it does to us today.
Excitingly, these findings are among the first science results of Euclid’s Early Release Observations, which represent just the first 24 hours of Euclid’s observations before it begins observing its main science targets, the billions of galaxies in more than a third of the sky, on February 14, 2024. .
The team’s research is featured on the paper’s arXiv repository page.