Astronomers observe intra-group light between galaxies

The brightest parts of the intra-group light are ~50 times fainter than the darkest night sky on Earth.

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An international team of astronomers used a novel method to characterize the stars that reside in a group of galaxies and the ‘intra-group light,’ a dim light between them.

The brightest parts of the intra-group light are ~50 times fainter than the darkest night sky on Earth. It is tough to detect, even with the largest telescopes on Earth — or in space.

Astronomers used their sensitive technique to eliminate light from all objects except intra-group light. They detected the intra-group light but were able to study and tell the story of the stars that populate it.

Dr. Cristina Martínez-Lombilla from the School of Physics at UNSW Science said, “We analyzed the properties of the intra-group stars — those stray stars between the galaxy groups. We looked at the age and abundance of the elements that composed them, and then we compared those features with the stars still belonging to galaxy groups.”

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

The orphan stars in the intra-group light were “anachronistic” and seemed to come from a different place than their nearest neighbors. According to the researchers, the character of the intra-group stars resembled the nebulous “tail” of a distant galaxy.

By combining these hints, the researchers were able to reconstruct the history—the tale—of the intra-group light and how its stars ended up in their own stellar orphanage.

Dr. Martínez-Lombilla said“We think these individual stars were at some points stripped from their home galaxies, and now they float freely, following the gravity of the group. The stripping, called tidal stripping, is caused by the passage of massive satellite galaxies — similar to the Milky Way — that pull stars in their wake.”

“This is the first time the intra-group light of these galaxies has been observed.”

“Unveiling the quantity and origin of the intra-group light provides a fossil record of all the interactions a group of galaxies has undergone and provides a holistic view of the system’s interaction history.”

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

“We have developed a tailored image treatment procedure that allows us to analyze the faintest structures in the Universe,” said Dr. Martínez-Lombilla.”

“It follows the standard steps for the study of faint structures in astronomical images — which implies 2D modeling and the removal of all light except that coming from the intra-group light. This includes all the bright stars in the images, the galaxies obscuring the intra-group light, and a subtraction of the continuum emission from the sky.”

“What makes our technique different is that it is fully Python-based, so it is very modular and easily applicable to different sets of data from different telescopes rather than being just useful for these images.”

“The most important outcome is that when studying very faint structures around galaxies, every step counts, and every undesirable light should be accounted for and removed. Otherwise, your measurements will be wrong.”

The techniques presented in this study are a pilot, encouraging future analyses of intra-group light.”

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

“This is key work for preparing the next generation of deep all-sky surveys such as those to be performed with the Euclid space telescope and the LSST with the Vera C. Rubin Observatory.”

Journal Reference:

  1. Cristina Martínez-Lombilla, Sarah Brough, et al. Galaxy And Mass Assembly (GAMA): extended intragroup light in a group at z = 0.2 from deep Hyper Suprime-Cam images. Monthly Notices of the Royal Astronomical Society, 2023; 518 (1): 1195 DOI: 10.1093/mnras/stac3119

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