Among the giant clusters of galaxies, stars wander, not gravitationally bound to a single galaxy. They give a ghostly glow. The big question is: where did they come from?

Astronomers have long wondered how these wandering stars can scatter across a cluster. There are several theories: The stars were very early: they formed several billion years ago in the cluster’s formative period. Alternatively, they may have been kicked out of a galaxy or tossed back and forth between galaxies after their convergence.

Infrared
A recent infrared survey from the good old Hubble sheds light on this issue. This NASA space telescope examined the so-called “light within the cluster” and concluded that the stars have been wandering around for billions of years and are not the product of more recent activity in a star cluster that has ejected them from their normal galaxies.

Hubble observed ten clusters located about 10 billion light-years from Earth. Since the light is 10,000 times dimmer than the night sky when seen from Earth, measurements can only be taken from space.

Homeless youth
The research revealed that the refraction of light within the cluster remains constant over several billion years, compared to the total amount of light in the cluster. “This means that these stars were indeed homeless in the early stages of cluster formation,” said James G of Yonsei University in Seoul, who presented his findings. Posted in nature.

Ghostlight / Hubble

Stars can scatter from their birthplaces as they orbit the cluster’s center as the galaxy moves through gaseous material in intergalactic space. In the process, gas and dust are pushed out of the galaxy. But this mechanism is not the main reason for the formation of stars within the cluster, according to the Hubble study. Then the refraction of light will increase over the years, but this is not the case. Light has remained unchanged for many billions of years.

Leaked stars
We don’t know exactly what makes stars homeless. Current theories can’t explain our results, but somehow they were mass-produced in the early universe,” says Gee. “In their early formative years, galaxies may have been very young and stars very easily leaked out because there was less gravity.”

“If we can determine the origin of the stars within the cluster, we will gain more knowledge about the formation history of a whole group of galaxies, and they can also serve as visible traces of the dark matter that surrounds the cluster,” said colleague Hyungjin Joo. . Dark matter is the invisible matter in the universe that holds galaxies and clusters together.

Play pinball
If wandering stars had formed during a relatively recent game of intergalactic pinball, they would not have had enough time to spread out over the entire gravitational field of the cluster and are therefore unsuitable for determining the distribution of dark matter. But if they formed in the early stages of star clusters, they were able to spread out completely. As a result, astronomers can use the stars to map the distribution of dark matter.

This technique is new and complements the traditional method of investigating where dark matter resides, that is, by gravitational lensing. This is a relatively strong gravitational field, like that of a galaxy, that bends light from an object behind it.

Since 1951
Intracluster light was first discovered in 1951 in the coma cluster. Because this group, which contains no fewer than 1,000 galaxies, is one of the closest clusters, about 330 million light-years from Earth, astronomer Fritz Zwicky was able to observe the ghostly light using a modest 18-inch telescope. . Hubble and the new James Webb telescope could better search for stars within the innermost cluster in the universe. Hopefully soon their mysterious existence will be fully explained.