Using the Euclid Space Telescope, scientists have discovered a whopping 1.5 trillion orphan stars floating through a massive cluster of thousands of galaxies, one of the largest structures in the cosmos.
These orphan 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 weak that it is many thousands of times darker than the night sky above Earth.
By observing this intracluster light in the Perseus cluster, which is located 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 where the origin of the cosmic orphans it emits.
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Euclid launched on July 1, 2023 from Cape Canaveral in Florida atop a SpaceX Falcon 9 rocket. Euclid’s main mission is to investigate dark energy, the mysterious force that accelerates the expansion of the universe, and dark matter, an “invisible” substance that does not. does not interact with light and is not made of atoms like the ‘everyday’ things that surround us.
Despite being designed to peer into the invisible ‘dark universe’, the telescope was also able to detect light coming from between galaxies in the Perseus cluster.
“We were surprised by our ability to see so far into the outer regions of the cluster and distinguish 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, brightness and configurations, we discovered that they come from small galaxies.’
Orphan stars have the blues
The key to understanding orphan stars in Perseus was Euclid’s ability to see the faintest light in the cluster, the intracluster light, which comes not from the galaxies, but from between them.
“This diffuse light is more than 100,000 times fainter than the darkest night sky on Earth,” said Matthias Kluge, team member and Max-Planck Institute for Extraterrestrial Physics. “But it’s spread out over such a large volume that, if we add it all up, it accounts for about 20% of the brightness of the entire cluster.”
The orphan stars that Euclid saw in the Perseus cluster can be distinguished by their characteristic blue color and their loose clustering. These features allowed Hatch and colleagues to trace their origins.
The team found that some of these free-wandering stars in intracluster space were being dragged from the outskirts of galaxies through interactions with other galaxies. Other orphan stars they found came from smaller dwarf galaxies in the Perseus cluster that have been completely disrupted.
What the team discovered next surprised them. Once ripped from their home galaxies, intracluster stars are expected to orbit the largest galaxies in the cluster they are isolated in, almost like a lost child at the mall gravitating toward the nearest adult.
However, Hatch and colleagues did not find this in Perseus with 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 University of Nottingham astronomer Jesse Golden-Marx. “This recent merger could have caused a gravitational disturbance, causing the most massive galaxy 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 star cluster. The diffuse light within the clusters appears to be distributed in a similar way to the globular clusters in Perseus, so these conglomerates of stars appear to be the source of at least some of this light.
The stars in these globular clusters lack high concentrations of “metals,” a term astronomers use for elements heavier than hydrogen and helium. This implies to the team that the globular clusters in the Perseus cluster moved in from the vast collection of outer galaxy edges, which are also ‘metal-poor’.
Globular clusters are a dominant feature in dwarf galaxies, meaning that some of the intracluster light may come from the remnants of such small galaxies that have been torn apart by tidal forces created during encounters with more massive galaxies.
The team also found from the Euclid observation of Perseus that the small dwarf galaxies in this galactic cluster increase in number as one moves away from the center of the cluster.
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The research helps verify Euclid’s ability to understand the evolution of galaxies and galaxy clusters and thus how the universe came to look the way it does to us today.
Excitingly, these findings are among the first scientific results from Euclid’s Early Release Observations, representing just the first 24 hours of Euclid’s observations before it began observing its main scientific targets: billions of galaxies spread across more than a third of the sky on February 14, 2024. .
The team’s research can be seen on the paper repository site arXiv.