There's a lot more out there in space than immediately meets the eye.
In the sparse chasms that yawn between the galaxies gravitationally bound in huge clusters, lone stars wander alone, like cosmic rōnin, misplaced and unconstrained.
We don't know where these stars came from. Were they kicked out from host galaxies? Or were they always there, forming alone in the darkness?
To find answers, the Hubble Space Telescope looked at 10 galaxy clusters, with light traveling for as long as 10 billion years to reach us. And the observations revealed that the feeble, diffuse light of lonely stars was glimmering in intracluster space in the early Universe; those stars have been roving alone for a very long time.
"This means that these stars were already homeless in the early stages of the cluster's formation," says astronomer James Jee of Yonsei University in South Korea.
This rules out recent gravitational interactions kicking galactic stars out of their homes and into intergalactic space.
Galaxy clusters are dynamic environments consisting of up to hundreds of thousands of galaxies, brought together by gravity. In such high-density situations, galaxies collide and merge at a higher rate, their gravitational dances pulling out long streams of material before the final coalescence occurs.
The tenuous gas between galaxies in a cluster, called the intracluster medium, can also introduce drag that strips material (and stars) out of a galaxy as it orbits the cluster center.
Different situations inside galaxies – such as lopsided supernovae and three-body gravitational interactions involving a black hole – can result in stars getting booted out into interstellar space at galactic escape velocities. This is unlikely to account for the intracluster light since these scenarios are expected to occur regardless of galactic cluster membership.
So there are three plausible origins for rogue intracluster stars: mergers, stripping, or the stars were already there when the cluster was forming.
We know that drag in the intracluster medium can't be responsible, Jee and his colleague, astronomer Hyungjin Joo of Yonsei University, found. That's because, over time, more and more stars would be pulled out of galaxies into intracluster space, increasing the proportion of intracluster light.
However, all across the history of the Universe, to 10 billion years ago, intracluster light remains steady. The researchers could find no evidence of an increasing glow.
"We don't exactly know what made [the stars] homeless. Current theories cannot explain our results, but somehow they were produced in large quantities in the early universe," Jee says. "In their early formative years, galaxies might have been pretty small and they bled stars pretty easily because of a weaker gravitational grasp."
Rather, the team says, their findings suggest that the dominant source of intracluster light is either a growth in tandem with the formation and growth of the brightest galaxies in the cluster when the cluster was still forming or by incorporating stray stars that were just floating around when the cluster was coming together.
This is important because the intracluster light can help map the invisible dark matter helping to bind the cluster together. If the galaxy mergers were recent, the stars ejected during this disruptive process would not have had time to disperse amongst the cluster space, and the light would not, therefore, accurately map the dark matter distribution.
Stray stars that were around from the cluster's earliest days, however, would be much more evenly distributed.
"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," Joo says.
The nature and role of dark matter in the Universe are still giant mysteries; mapping the ghostly light between galaxies could help us solve them.
The research has been published in Nature.