Diffuse light from stars gravitationally bound to the halo of galaxy clusters rather than specific galaxies is known as intracluster light (ICL). The ICL fraction, determined by the ratio of the ICL to the total light, is expected to decrease rapidly with increasing redshift to a level of a few percent, according to leading theories. Because only two clusters have been studied in this redshift domain, observational studies have yet to determine the relationship outside the redshift unit.
The nagging question for astronomers was: How did the stars get so scattered throughout the cluster in the first place?
A recent infrared survey from NASA’s Hubble Space Telescope, which searched for this so-called “intracluster light,” sheds new light on the mystery. According to the latest Hubble findings, these stars are not the result of more recent dynamic activity within a cluster of galaxies, which would have removed them from typical galaxies, but have been wandering for billions of years.
The study included ten clusters of galaxies nearly 10 billion light-years away. The faint light within the cluster is 10,000 times fainter than the night sky when viewed from the ground. Therefore, these measurements must be taken from space.
It was found that the fraction of the light within the cluster relative to the total light within the cluster remains constant, looking back billions of years in time. It suggests that these stars were already homeless in the early stages of the cluster’s formation.
James Jee of Yonsei University in Seoul, South Korea, said: “Stars can be scattered beyond their galactic birthplaces when a galaxy moves through gaseous material in the space between galaxies as it orbits the center of the cluster. In the process, drag pushes gas and dust out of the galaxy.” however, the new Hubble study excludes that this mechanism is the primary cause of star production within the cluster, because the fraction of light within the cluster would increase over time if stripping is the major player, but it is not the case in the new Hubble data, which show a constant fraction over billions of years.”
“We don’t know exactly what caused them to become homeless. Current theories cannot explain our results, but they were produced in large quantities in the early universe. In their early formative years, galaxies may have been small, and they bleed stars easily because of weaker gravity.”
Hyungjin Joo of Yonsei University, the first author of the paper, said: “If we pinpoint the origins of stars within the cluster, it will help us understand the assembly history of an entire cluster of galaxies, and they could serve as visible tracers of dark matter enveloping the cluster.”
The wandering stars wouldn’t be able to trace the distribution of the cluster’s dark matter if they were created by a very recent game of pinball between galaxies, because they didn’t have enough time to spread throughout the cluster’s gravitational field. However, the stars will have spread completely through the cluster if they were born in the early years of the cluster. This would allow astronomers to map the cluster’s distribution of dark matter using the wandering stars.
By observing how the entire cluster distorts light from background objects due to a phenomenon known as gravitational lensing, this method is new and adds to the conventional way of dark matter mapping.
- Joo, H., Jee, MJ Intracluster light is already abundant at redshift beyond unity. Nature 613, 37-41 (2023). DOI: 10.1038/s41586-022-05396-4