Astronomers are learning to decipher clues imprinted in the ultra-faint light of free-floating stars that have been kicked out of their galaxies and left to wander the vast interstitial realms of space.
This ambient glow, known as intracluster light because it shines between galaxies in clusters, is a powerful new way to probe longstanding mysteries, such as the nature of dark matter, an unidentified substance that is abundant in the universe.
While intracluster light has been known about for decades, it is so faint that only recent and more advanced telescopes have been able to dig into this unique cosmic “fossil record,” a shift that heralds a new “era of deep and wide surveys that promise to uncover the faint universe,” according to a review article published on Monday in Nature Astronomy.
This eerie intracluster glow is “not very well-studied or famous,” said Mireia Montes, an STScI Prize Fellow at the Space Telescope Science Institute in Baltimore, in a call. “What we want with these surveys is to see how this light evolves and grows with time, and how it relates to the mass of the cluster.”
These clusters can contain hundreds or thousands of galaxies that are all gravitationally bound together. Stars in these clusters can be ejected from host galaxies by a variety of processes.
Sometimes, small galaxies are torn apart by their larger neighbors, spilling their stellar innards across the cluster. Sometimes, stars are catapulted into deep space by tumultuous collisions between large galaxies like the Milky Way. Whatever their origin, the free-floating stars in clusters are not individually visible to astronomers, but the sum total of their light contains information about their backstories.
“When galaxies that are very close to each other experience encounters, they have these gravitational and tidal forces that strip stars from those galaxies,” Montes said. “These stars end up going into this place, this medium, between the galaxies. They do not belong to any galaxy anymore; they float around.”
“Depending on what has happened in the cluster, the stars that go into the intracluster light come from one place or another, and they have a signature, a characteristic color, that tells us where they came from,” she added. “Basically, by studying this color, I can determine the origin of this light and what happened in the cluster before.”
For instance, certain colors of light are associated with lighter elements, which could suggest the presence of more ancient stars that haven’t evolved heavier metals. Intracluster light associated with more metal-rich stars could signal that younger systems were pulled into this galactic no-man’s land. As a result, the intracluster glow can yield insights about the past dynamical interactions within galaxy clusters that would be otherwise inaccessible.
Moreover, this faint light opens a new window into dark matter, a type of invisible matter that provides the scaffolding of our universe, yet is very poorly understood because it does not emit light. Scientists are only able to study dark matter by observing its gravitational influence on luminous objects, such as stars and galaxies.
Intracluster stars between galaxies are proving to be especially useful in exposing the distribution of dark matter in these clusters, revealing the shadowy silhouette of the mysterious substance.
The fact that the light follows “the potential of the cluster” and its distribution extends “everywhere in the cluster” makes it a “good tracer for dark matter,” Montes explained.
“Where the dark matter dominates, the stars dominate,” she continued. The light “follows what dark matter is doing. In this sense, the stars have a similar property that dark matter has, which is that they don’t interact in any other way than gravitationally. The only thing is that they are really faint so you have to put a lot of time in the telescope to see them.”
Indeed, studying intracluster light can be meticulous work because it is so elusive. But next-generation observatories, such as the Vera Rubin Observatory or the Nancy Grace Roman Space Telescope, will be able to capture more of this murky starlight to advance this emerging field of study. As Montes notes in her study, “we are still only scratching the (bright) surface of what we can learn from this light.”