Driven from their homes, they roam clusters that span thousands of galaxies. Studying the “ghostly light” they emit could unlock one of the great mysteries of the universe.
There are stars that, according to NASA, “float like lost souls” in the universe. And the light they emit is so faint that the US space agency calls it a “ghostly nebula.”
We are talking about stars that, unlike the most wellknown, do not live in a galaxy. They’ve been wandering through clusters that bring thousands of galaxies together for billions of years, according to a new study using images from the Hubble Space Telescope.
But how were wandering stars torn from their host galaxies?
Studying these “lost souls” is important, according to Spanish astronomer Mireia Montes of the Canary Islands Institute of Astrophysics.
Montes studies the faint light emitted by wandering stars, called intracluster light. She told BBC News Mundo, the BBC’s Spanishlanguage service, that this soft glow can reveal not only the structure of galaxy clusters, but also the nature of one of the universe’s greatest mysteries: dark matter.
What are wandering stars?
Montes explains that “in galaxy clusters, which are the largest gravitybound structures, galaxies there may be hundreds to thousands of them are found in astronomically small spaces”.
The scientist claims that because the galaxies are so close together, they interact gravitationally. And with these interactions, some stars are thrown out of position and eventually end up in intergalactic space.
Montes compares these interactions to tidal forces between the earth and the moon. “When you feel the power of the tides on Earth, you don’t see much except the rise of the sea. But in the case of galaxies that are not solid, these forces pull the stars out of the galaxies. “
Over time, the interactions produce a very diffuse light that we call intracluster light.
“I liken it to writing on the blackboard with chalk while maintaining the proportions,” explains Montes. “This dust is gradually released thanks to the friction of the chalk with the slate.”
Intracluster light image of cluster SMACSJ0723.37327 taken with the NIRCAM camera on board the James Webb Space Telescope and processed by Mireia Montes and Ignacio Trujillo from the Institute of Astrophysics of the Canary Islands (Spain). Galaxies are highlighted in color and the soft light between galaxies in black and white
Image: NASA, ESA, CSA, STScI and Mireia Montes (IAC)
The soft light of wandering stars
According to Montes, the wandering stars mostly resemble our Sun. But because they are so widespread, they shine very faintly, accounting for about 1% or less of the brightness of the darkest sky we see on Earth.
“When we go to a professional observatory, like in Chile or the Canary Islands, the sky is darker because we always try to avoid light pollution,” she explains. “Imagine light making up 1% or less of this very dark sky!”
The new Hubble data study was based on 10 galaxy clusters located nearly 10 billion lightyears from Earth. The research found that the fraction of intracluster light relative to total cluster light has remained constant for billions of years.
This means that “these stars were already shifting in the early stages of cluster formation,” according to James Jee of Yonsei University in Seoul, South Korea, who was one of the study’s authors.
Spanish astronomer Mireia Montes of the Canary Islands Institute of Astrophysics uses very deep images of the Universe to study the formation and evolution of galaxies. She searches for clues to the mysterious dark matter
Image: Mireia Montes
What does this light reveal about dark matter?
Mireia Montes explains that examining the properties of the light within the cluster, as well as the ages of the stars and the amount of metals they contain, provides information about the history of the structure.
“What we study in the sky is very static, we only have photographs of that moment of what is happening to the study object,” she says. “But when we examine the light within the cluster, it’s like having a document from the cluster’s past.”
Intracluster light also provides clues to mysterious dark matter. Dark matter is estimated to make up about 25% of all matter in the cosmos, but cannot be observed directly because it does not absorb, reflect, or emit light.
But scientists know it exists because dark matter acts on observable objects.
In 1997, a Hubble image showed how light from a distant galaxy cluster bends as it passes through another cluster in front. This effect is known as gravitational lensing.
The scientists estimated that the mass of the cluster in the foreground of the image would have to be 250 times larger than the visible matter to bend light in this way. They believe that dark matter is at the root of this unexplained mass.
The SDSS cluster J1038+4849 in an image captured by Hubble. What looks like a smiley face in the center of the image, the two “eyes” are very bright galaxies and the “smile” is an arc caused by gravitational lensing. Cluster distorts and bends light from objects behind it
In the case of wandering stars, Montes and other researchers showed in 2019 that the light from these stars follows the distribution of dark matter in galaxy clusters.
“Remember that these stars are not associated with galaxies that form a diffuse and extended light,” explains the astronomer. “In addition, they sway according to the gravity of the heap. In these structures there is about 300 times more dark matter than stars.”
“This is why light tracks dark matter so well, because it lives in the parts of the cluster where dark matter is dominant.” And that, according to Montes, has important implications.
Image of the Abell 85 cluster taken with the Hyper SuprimeCam camera on the Subaru telescope on Mauna Kea, Hawaii, processed by Mireia Montes and her collaborators
Image: Mireia Montes
“Usually we know how mass is distributed in clusters because they are like gravitational lenses,” explains the astronomer. “That means they deform and increase the brightness of whatever is behind them. Because of this, humans tend to use them to observe the distant universe, as they make observing these very distant galaxies easier.”
“But if we want to know the true properties of these objects, we need to know what this lens, the galaxy cluster, looks like and how it distorts the real image of the distant galaxy.”
This requires more complex techniques such as modeling and spectroscopy. But with intracluster light it is only necessary to take a “very deep image” of these objects.
The James Webb Space Telescope is expected to make great strides in studying faint light within clusters
Montes points out that there is still a lot we don’t know about the light within clusters: how it evolves over time and how the mass of the cluster is related to the amount of light, for example.
Another problem, according to the scientist, is how the mass distribution in galaxy clusters is related to the very nature of dark matter.
The James Webb Space Telescope, whose first images were released in July 2022, is expected to make major strides in studying intracluster light.
“We believe James Webb will be revolutionary in many aspects of astronomy, and in this case intracluster light,” Montes said. She points out that the James Webb telescope is larger than the Hubble telescope and therefore more efficient for observing fainter objects because it can collect more light in less time.
This text was published at https://www.bbc.com/portuguese/geral64308461