Clusters of galaxies often have hundreds of members
packed together in a relatively small region of the universe. But even in the
dark region between these galaxies, the intracluster space, there is a faint
glow. This is produced by wandering stars that don’t belong to any galaxy. How
they got there is a mystery – but new Hubble observations seem to have pointed
out something very important.
Possibilities for the origin of these stars usually
see calm or violent interactions between galaxies. Mergers can certainly throw
stars into wider orbit or even kick them out of the final galaxy altogether.
Alternatively, they could have been pulled out of a galaxy due to gravitational
interaction over time.
However, observations from Hubble of 10 different
clusters at different ages of the Universe show that the amount of intracluster
light emitted has not changed much in the last 10 billion years. Those stars
have been wandering without host galaxies for a very long time.
"This means that these stars were already
homeless in the early stages of the cluster's formation," co-author James
Jee of Yonsei University in Seoul, South Korea, said in a statement. "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
universe. In their early formative years, galaxies might have been pretty small
and they bled stars pretty easily because of a weaker gravitational grasp."
If the stars were ripped from their galaxies due to
mergers, they wouldn’t have time to scatter across the whole cluster. If it was
due to regular stripping, the amount of light should be increasing over time,
not staying the same. It's likely they were already in place billions of years
ago and not many were added to them over time.
"If we figure out the origin of intracluster
stars, it will help us understand the assembly history of an entire galaxy
cluster, and they can serve as visible tracers of dark matter enveloping the
cluster," added Hyungjin Joo of Yonsei University, the first author of the
paper.
Intracluster light is another approach to studying
the distribution of dark matter. This is a hypothetical substance that holds
galaxies together in a certain shape but it is invisible. By using the
distribution of this light, one can trace where dark matter is. Future studies
might also employ JWST, which has recently demonstrated its capabilities at
observing intracluster light.
Reference: Nature
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