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Mysterious Intra-Group Light Unraveled by Astronomers

An international team of astronomers has used a novel technique to characterize the stars that reside in the ‘intra-group light,’ or faint light that exists between galaxies.

Image Credit: suns07butterfly/

We know almost nothing about intra-group light. The brightest parts of the intra-group light are ~50 times fainter than the darkest night sky on Earth. It is extremely hard to detect, even with the largest telescopes on Earth—or in space.

Dr. Cristina Martínez-Lombilla, Researcher, School of Physics, University of New South Wales

The researchers were able to not only detect the intra-group light but also study and narrate the history of the stars that populate it with their sensitive technique, which filters out all light from objects except that from the intra-group light.

We analyzed the properties of the intra-group stars—those stray stars between the galaxies. 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,” Dr. Martínez-Lombilla stated.

Rebuilding the Story of Intra-Group Light

The orphan stars in the intra-group light were not only “anachronistic,” but they also seemed to come from a different place than their nearest neighbors. The character of the intra-group stars, according to the researchers, resembled the nebulous “tail” of a distant galaxy.

The researchers were able to piece together these hints to reconstruct the intra-group light’s history and the tale of how its stars ended up in their own stellar orphanage.

Dr. Martínez-Lombilla further added, “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.

The intra-group light of these galaxies has been observed for the first time.

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,” Dr Martínez-Lombilla further added.

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

The researchers are providing crucial data points to the slow-burning evolution of cosmic events by observing events that occurred a long time ago in galaxies that are extremely far away.

Tailored Image Treatment Procedure

To achieve this insightful perspective, the researchers invented an original technique.

We have developed a tailored image treatment procedure that allows us to analyze the faintest structures in the Universe. It follows the standard steps for the study of faint structures in astronomical images—which implies 2D modelling 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,” further added Dr Martínez-Lombilla.

Dr. Martínez-Lombilla pointed out, “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 in the process counts and every undesirable light should be accounted for and removed. Otherwise, your measurements will be wrong.

Dr. Martnez-Lombilla stated that the methods utilized in this research are a pilot, boosting future analyses of intra-group light.

Our main long-term goal is to extend these results to a large sample of 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,” Dr. Martnez-Lombilla added.

Dr. Martnez-Lombilla concluded, “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:

Martnez-Lombilla, C., et al. (2022) Galaxy and Mass Assembly (GAMA): extended intragroup light in a group at z = 0.2 from deep Hyper Suprime-Cam images Get access Arrow. Monthly Notices of the Royal Astronomical Society. doi:10.1093/mnras/stac3119

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