Astronomers observe intragroup light – the elusive glow between distant galaxies – Zoo House News

Astronomers observe intragroup light – the elusive glow between distant galaxies – Zoo House News

  • Science
  • November 24, 2022
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An international team of astronomers has applied a new technique to a group of galaxies and the faint light between them – known as “intragroup light” – to characterize the stars that live there.

The lead author of the study published in MNRAS, Dr. Cristina Martínez-Lombilla, from UNSW Science’s School of Physics, said: “We know almost nothing about intragroup light.

“The brightest parts of the intragroup light are about 50 times fainter than Earth’s darkest night sky. It’s extremely difficult to see, even with the largest telescopes on Earth — or in space.”

Using their delicate technique that eliminates light from all objects except the intragroup light, the researchers not only discovered the intragroup light, but were also able to study and tell the story of the stars that populate it.

“We analyzed the properties of the intragroup stars – those stray stars between the galaxy groups. We looked at the ages and abundances of the elements that make them up, and then we compared those features to the stars that still belong to galaxy groups,” said Dr. Martínez-Lombilla.

“We found that the light within the group is younger and less metal-rich than that of the surrounding galaxies.”

Reconstructing the history of the group’s internal light

The orphaned stars in the intragroup light were not only “anachronistic,” but appeared to have a different origin than their nearest neighbors. The researchers found that the character of the stars within the group resembled the nebulous “tail” of a more distant galaxy.

Combining these clues allowed the researchers to reconstruct the story — the story — of the intragroup light and how its stars were collected in their own stellar orphanage.

“We believe that these individual stars have been removed from their home galaxies in some places and are now floating freely by following the group’s gravity,” said Dr. Martínez-Lombilla. “The stripping, called tidal stripping, is caused by the passage of massive satellite galaxies – similar to the Milky Way – pulling stars in their wake.”

This is the first time that intragroup light from these galaxies has been observed.

“Revealing the amount and origin of intragroup light provides fossil evidence of all interactions that a group of galaxies has undergone and provides a holistic view of the system’s interaction history,” said Dr. Martínez-Lombilla.

“Furthermore, these events happened a long time ago. The galaxies [we’re looking at] are so far away that we are observing them as they looked 2.5 billion years ago. That’s how long it takes for their light to reach us.”

By observing events long ago in galaxies this far away, researchers are contributing important data points to the slow-burning evolution of cosmic events.

Customized image processing method

Researchers developed a unique technique to achieve this penetrating vision.

“We have developed a tailor-made image processing method that allows us to analyze the faintest structures in the universe,” said Dr. Martínez-Lombilla.

“It follows the standard steps for examining faint structures in astronomical images – which involves 2D modeling and removal of all but the light within the cluster. This includes any bright stars in the images, the galaxies that obscure the intragroup light, and a subtraction of the continuum emission from the sky.

“What sets our technique apart from others is that it’s entirely Python-based, so it’s very modular and easily applicable to different datasets from different telescopes, rather than just being useful for those images.”

“The most important finding is that when studying very faint structures around galaxies, every step in the process counts and any unwanted light should be considered and removed. Otherwise your measurements will be wrong.

The techniques presented in this study are a pilot project and encourage future analysis of the group’s internal light, said Dr. Martínez-Lombilla.

“Our main long-term goal is to extend these results to a large sample of galaxy groups. Then we can look at statistics and find out the typical properties regarding the formation and evolution of the light within the group and these extremely common groups of galaxies.

“This is key work in preparation for the next generation of deep all-sky surveys such as will be conducted with the Euclid Space Telescope and the LSST with the Vera C. Rubin Observatory.”

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