Free Astronomy Magazine March-April 2023

47 MARCH-APRIL 2023 T hese are Hubble Space Telescope images of two massive clus- ters of galaxies named MOO J1014+0038 (left panel) and SPT-CL J2106-5844 (right panel). The artificially added blue color is trans- lated from Hubble data that captured a phenomenon called intra- cluster light. This extremely faint glow traces a smooth distribution of light from wandering stars scattered across the cluster. Billions of years ago the stars were shed from their parent galaxies and now drift through intergalactic space. [NASA, ESA, STScI, James Jee (Yonsei University). Image processing: Joseph DePasquale (STScI)] the mystery. The new Hubble obser- vations suggest that these stars have been wandering around for billions of years, and are not a product of more recent dynamical activity in- side a galaxy cluster that would strip them out of normal galaxies. The survey included 10 galaxy clus- ters as far away as nearly 10 billion light-years. These measurements must be made from space because the faint intracluster light is 10,000 times dimmer than the night sky as seen from the ground. The survey reveals that the fraction of the intracluster light relative to the total light in the cluster remains constant, looking over billions of years back into time. “This means that these stars were already home- less in the early stages of the clus- ter’s formation,” said James Jee of Yonsei University in Seoul, South Korea. His results have being pub- lished in the January 5 issue of Na- ture magazine. Stars can be scattered outside of their galactic birthplace when a galaxy moves through gaseous mate- rial in the space be- tween galaxies, as it orbits the center of the cluster. In the process, drag pushes gas and dust out of the galaxy. However, based on the new Hubble survey, Jee rules out this mechanism as the primary cause for the intracluster star pro- duction. That’s because the intracluster light fraction would increase over time to the present if stripping is the main player. But that is not the case in the new Hubble data, which show a constant frac- tion over billions of years. “We don’t exactly know what made them homeless. Current theories cannot explain our results, but somehow they were produced in large quantities in the early uni- verse,” said Jee. “ In their early form- ative years, galaxies might have been pretty small and they bled stars pretty easily because of a weaker gravitational grasp.” “If we figure out the origin of intra- cluster stars, it will help us under- stand the assembly history of an entire galaxy cluster, and they can serve as visible tracers of dark mat- ter enveloping the cluster,” said Hyungjin Joo of Yonsei University, the first author of the paper. Dark matter is the invisible scaffold- ing of the universe, which holds galaxies, and clusters of galaxies, to- gether. If the wandering stars were produced through a comparatively recent pinball game among galax- ies, they do not have enough time to scatter throughout the entire gravitational field of the cluster and therefore would not trace the distri- bution of the cluster’s dark matter. But if the stars were born in the cluster’s early years, they will have fully dispersed throughout the clus- ter. This would allow astronomers to use the wayward stars to map out the dark matter distribution across the cluster. This technique is new and comple- mentary to the traditional method of dark matter mapping by measur- ing how the entire cluster warps light from background objects due to a phenomenon called gravita- tional lensing. Intracluster light was first detected in the Coma cluster of galaxies in 1951 by Fritz Zwicky, who reported that one of his most interesting dis- coveries was observing luminous, faint intergalactic matter in the clus- ter. Because the Coma cluster, con- taining at least 1,000 galaxies, is one of the nearest clusters to Earth (330 million light-years), Zwicky was able to detect the ghost light even with a modest 18-inch telescope. NASA’s James Webb Space Tele- scope’s near-infrared capability and sensitivity will greatly extend the search for intracluster stars deeper into the universe, and therefore should help solve the mystery. !