Astronomers have used NASA’s Hubble Space Telescope to shed light on the mystery of wandering star formation.
Wandering stars that are not gravitationally bound to any galaxy in a cluster can be found in clusters of hundreds or thousands of galaxies. These stars wander among galaxies emitting a ghostly haze of light. For several years, astronomers have wondered how these wandering stars become so spread out across the cluster. Leading theories include the possibility that the stars were ejected from the cluster’s galaxies, or were tossed around after a galaxy merger, or that they were present early in the cluster’s formative years many billions of years ago.
A recent infrared survey by NASA’s Hubble Space Telescope shed light on the mystery, with new observations suggesting that these wandering stars have been around for billions of years.
The study, “Inner cluster light already abundant at redshift above unity,” is published in the journal Nature.
Discoveries from the Hubble probe
The study involved ten galaxy clusters nearly ten billion light-years away. The measurements had to be made from space because the intracluster light is 10,000 times fainter than the night sky as seen from Earth.
It was revealed that the fraction of intracluster light relative to the total light in the cluster remains constant looking billions of years back in time. “This means that these stars were already homeless in the early stages of cluster formation,” said James Gee of Yonsei University in Seoul, South Korea.
Stars can be scattered outside their galactic birthplace when a galaxy moves through gaseous material in intergalactic space as it orbits the cluster center. During this process, drag pushes gas and dust out of the galaxy.
© NASA, ESA, STScI, James Gee (Yonsei University); Image processing: Joseph DePasquale (STScI)
However, the results of the new Hubble study mean that this mechanism can be ruled out as the main cause of intracluster star production, as the intracluster light fraction would increase over time if stripping was the main player. New Hubble data show a constant fraction over billions of years.
“We don’t know exactly what made them homeless. Current theories cannot explain our results, but somehow they were produced in large quantities in the early universe,” Gee said. “In their early formative years, galaxies may have been quite small and shed stars quite easily due to weaker gravitational pull.”
The importance of discovering the origin of wandering stars
“If we understand the origin of intracluster stars, it will help us understand the assembly history of an entire galaxy cluster, and they can serve as visible markers of the dark matter surrounding the cluster,” said Hyungjin Joo of Yonsei University, first author of the paper. Dark matter holds galaxies and galaxy clusters together.
If the wandering stars were produced by a relatively recent game of pinball among the galaxies, they would not have had enough time to disperse throughout the cluster’s gravitational field. Therefore, they will not trace the dark matter distribution of the cluster. However, if the stars were born in the early years of the cluster, they would have been completely dispersed throughout the cluster. Astronomers could then use the wandering stars to map the distribution of dark matter in the cluster.
This new technique complements the traditional method of mapping dark matter by measuring how the entire cluster distorts light from background objects due to a phenomenon called gravitational lensing.
Detection of intracluster light
Fritz Zwicky first discovered intracluster light in 1951 in the Coma cluster of galaxies. Because this cluster is one of the closest clusters to Earth, the ghostly light was detected by Zwicky with a modest 18-inch telescope.
The near-infrared capability and sensitivity of NASA’s James Webb Space Telescope will deepen the search for intracluster stars deeper into the Universe and should therefore help solve the mystery.
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