In the vast clusters of galaxies in the Universe, which can consist of hundreds or thousands of galaxies, countless “rogue” stars wander among them. These stars are not gravitationally bound to a single galaxy, but rather to the halo of galaxy clusters themselves, and are only discernible by the diffuse light they emit, known as ghost light or intracluster light (ICL). For astronomers, the explanation for how these stars became so scattered across their galaxy clusters has always been an unsolved question.
There are several theories, including the possibility that the stars were pulled from their galaxies, expelled in galactic mergers, or were part of their cluster since early formation billions of years ago. Using NASA’s Hubble Space Telescope, a team from Yonsei University, Seoul, and the University of California, Davis, conducted an infrared survey of distant galaxy clusters. Their observations suggest these wandering stars have been drifting for billions of years and have not been removed from their respective galaxies.
The survey was conducted by Hyungjin Joo, a Ph.D. Yonsei Observable UNIverse Group (YOUNG) Candidate at Yonsei University, and Myungkook James Jee, YOUNG Group Leader and LSST Fellow at UC Davis. As they suggest in their recent article in the journal Nature, leading theories about ICL predict that its light fraction (compared to the total light of a galaxy cluster) decreases rapidly with increasing redshift, reaching only a few percent for clusters with redshift values of z >1 (about 10 billion light years away)
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NASA’s Chandra X-ray Observatory captured this image of the Coma galaxy cluster. Photo credit: NASA/CXC/Univ. from Chicago/I. Zhuravleva et al./SDSS
The first case in which ICL was discovered was in 1951 by the Swiss astronomer Fritz Zwicky, who reported observing luminous material while observing the Coma Cluster. This cluster contains at least 1,000 galaxies and is one of the closest to Earth, only 330 million light-years away. However, observational studies of ICLs with galaxy clusters up to 10 billion light-years away have been inconclusive because the ICL is so faint. In fact, the ICL of these galaxy clusters is about 10,000 times dimmer than the night sky when observed from Earth.
For their study, Jee and Joo examined deep-infrared image data collected by Hubble from 10 galaxy clusters with redshift values of 1 ? z? 2 (about 7.731 to 10.324 billion years away). Their results showed that the ICL light fraction remains constant over billions of years. As Jee explained in a recent NASA press release:
“This means that these stars were already homeless in the early stages of star cluster formation. We don’t know exactly what made her homeless. Current theories cannot explain our results, but somehow they were produced in large quantities in the early Universe. In their early formative years, galaxies might have been quite small and bleed out stars fairly easily due to a weaker gravitational pull.”
The theory that intercluster stars could be removed from the galaxies in which they were born is certainly not unfounded. As galaxies move through intercluster space while orbiting the center of a cluster, they pass gaseous materials. This creates drag that pushes gas and dust out of the galaxy into the intercluster space, providing the raw material for new star formation. However, based on their survey, Joo and Jee ruled out this mechanism as the main cause of the production of stars within the cluster, as it would mean that the ICL fraction would increase over time as the population grows.
Galaxy cluster SMACS 0723 seen by NIRCam on JWST shows many galaxies and gravitational lenses. Source: NASA/ESA/CSA/STScI
A fraction of light that is constant over billions of years indicates that something must have allowed many stars to be born billions of years ago. In any case, this will have immense implications for the study of galaxy formation and evolution, especially when it comes to dark matter questions. “Finding the origin of intracluster windows helps us understand the formation history of an entire galaxy cluster, and they can serve as visible traces of dark matter surrounding the cluster,” Joo said.
If stellar populations between star clusters were the result of stars being ejected from their galaxies during mergers, the stars would not have enough time to disperse throughout the cluster’s gravitational field. As a result, they would not track the cluster’s dark matter distribution. But if the stars formed in the cluster’s early history (i.e., billions of years ago), they would have dispersed throughout the cluster, allowing astronomers to use them to map the cluster’s DM distribution.
This new technique complements the traditional method of DM mapping known as gravitational lensing. With this technique, astronomers observe how the gravitational pull of an entire cluster changes the curvature of spacetime around it, distorting and enhancing light from background objects. These studies will also benefit from the near-infrared capability and sensitivity of NASA’s James Webb Space Telescope (JWST), which will greatly expand the search for intracluster stars in very high redshift galaxy clusters.
In turn, these investigations will provide new ways to measure the DM distribution and the role (if any) in the formation of the first galaxies in the Universe. From this, scientists can finally solve one of the greatest mysteries of modern astronomy and cosmology.
Further Reading: NASA, Nature