An article (link to the file in PDF format) published in the journal “Nature” reports the results of a study on the so-called intracluster light that permeates galaxy clusters. Hyungjin Joo and M. James Jee of Yonsei University in Seoul, South Korea, used the Hubble Space Telescope to examine ten galaxy clusters and the glow within them. The surprising and therefore interesting discovery was that intracluster light is abundant even in the oldest clusters, a sign that the stars that emit it were ejected from their galaxies a long time ago. This suggests that this happened at the same time as the formation and growth of the clusters.
The so-called intracluster light permeates galaxy clusters but is very dim, at brightness levels estimated at around 1% of that of the darkest sky observable from Earth. The image (Science: NASA, ESA, STScI, James Jee (Yonsei University). Image processing: Joseph DePasquale (STScI)) shows the galaxy clusters MOO J1014+0038 (left panel) and SPT-CL J2106-5844 (right panel) as seen by the Hubble Space Telescope’s Wide Field Camera 3 instrument at infrareds. Intracluster light is shown in blue.
The intracluster light measurements conducted by Hyungjin Joo and M. James Jee indicate that the ratio between the brightness given by the intracluster light and the total brightness of a galaxy cluster is almost identical for all the clusters examined. That’s true even for those up to ten billion light-years away, a result that indicates that the wandering stars that emit intracluster light were ejected from their galaxies when they were young.
This result offers confirmation to one of the hypotheses concerning the ejection mechanism of stars that emit intracluster light. Some mechanisms that see that ejection as a result of galaxy mergers or passing through clouds of intergalactic gas should cause the number of those stars to increase over time. In that case, a cluster we see as it was ten billion years ago should have dim intracluster light while a cluster we see as it was a billion years ago should have much brighter intracluster light. Seeing very similar intracluster light in all clusters indicates that the wandering stars were ejected as the clusters were forming and growing.
The existing models offer no explanation for the results obtained by the two researchers. Perhaps the first galaxies “lost” stars more easily because they were on average smaller in a mechanism that lasted relatively little because galaxy mergers generated galaxies that were on average far more massive. Even today there are dwarf galaxies but according to the researchers, in the recent universe, they don’t have a significant influence on intracluster light.
Intracluster light is studied in particular to trace the distribution of dark matter, and for this reason, observations with the James Webb Space Telescope have recently begun. If wandering stars spread across cluster galaxies billions of years ago, they’re well distributed in intergalactic space and the mapping will be more accurate. In short, understanding the mechanisms that lead stars to get lost in intergalactic space is important for studies on dark matter. For these reasons, studies of these orphan stars will continue.
