Baryons at the Edge: SRG/eROSITA Survey Detects “Missing” Cosmic Gas at the Outskirts of Galaxy Clusters

A team of astronomers at the Max Planck Institute for Extraterrestrial Physics (MPE), has detected hot gas extending beyond galaxy clusters using data from the eROSITA All-Sky Survey. This finding reveals the distribution of hot gas in the outskirts, indicating that galaxy clusters are actively accreting material from the cosmic web. This study shows that these regions host the baryonic matter that is missing from the galaxy cluster center, enhancing our understanding of galaxy cluster growth and the surrounding intergalactic environment.

Using data from the SRG/eROSITA All-Sky Survey, the team of international researchers have now achieved a key advance in tracing ordinary matter in the Universe. They detected hot, shock-heated gas extending far beyond the previously studied boundaries of galaxy clusters, offering a new perspective on how these vast cosmic structures grow by drawing in material from the surrounding intergalactic medium.

The study, led by scientists at the Max Planck Institute for Extraterrestrial Physics (MPE), focuses on the outermost regions of galaxy clusters – areas that have been particularly difficult to observe until now. The results reveal how hot gas is distributed in and around these distant outskirts, offering insights into the environments surrounding some of the most massive structures in the Universe.

Bridging the Gap Between Clusters and the Cosmos

Galaxy clusters are among the largest gravitationally bound systems in the Universe, containing hundreds to thousands of galaxies embedded within vast halos of dark matter and filled with hot, diffuse plasma. Yet, the transition between a cluster and the surrounding cosmic web – the network of gas filaments connecting large-scale structures – has long remained uncertain.

Over the past five decades, X-ray space telescopes have shown that galaxy clusters host hot thermal atmospheres with temperatures of tens of millions of degrees and spatial extents of several million light-years. However, the true size of these atmospheres has been unclear because their X-ray brightness drops sharply at large distances from the cluster center.

By “stacking” X-ray data from 680 galaxy clusters, the team amplified the faint glow of gas in these remote regions. They detected a statistically significant X-ray signal extending out to 4.5 megaparsecs (about 14 million light-years) – well beyond the virial radius, which is generally considered the cluster’s edge.

“The survey’s observation depth for a single object is shallow, but it covers the entire western Galactic hemisphere. By selecting 680 galaxy clusters in the nearby Universe, we obtained an extremely high signal-to-noise surface brightness profile through stacking,” explains lead author Xiaoyuan Zhang, postdoctoral researcher at MPE.

“Historically, observations have focused mainly on cluster centers because signals from the outskirts are weak. It is extremely exciting that we can now probe the very edges of clusters – regions that can tell us much about the fundamental physics of gas and dark matter,” adds co-author Benedikt Diemer, Assistant Professor at the University of Maryland.  

Using the IllustrisTNG cosmological simulations, developed by researchers at the Max Planck Institute for Astrophysics, the team showed that gas around galaxy clusters is not distributed evenly. It is much denser along cosmic filaments – the large-scale structures connecting matter across the Universe – than in the low-density voids between them. This indicates that galaxy clusters are actively accreting material from the cosmic web through these filamentary channels.

MPE research group leader Esra Bulbul, second author of the study, adds: “Astronomers have long searched for the Universe’s ‘missing baryons’ – the normal matter that should exist but has been difficult to detect. Our results show that, in the far outskirts of galaxy clusters, the amount of gas reaches about 90 percent of what we expect based on the Universe’s average matter density. This suggests that much of the ‘missing’ matter is indeed present, hidden in these vast, hot, and turbulent outer regions. This helps us understand not only how clusters grow but also the physics of the gas that fills the cosmos.”

This study highlights that, in addition to its strong source-detection capabilities, the eROSITA All-Sky Survey also enables the exploration of extremely faint emission – down to below one percent of the sky background – through stacking techniques.
 

eROSITA

The eROSITA instrument (extended ROentgen Survey with an Imaging Telescope Array) is the primary telescope aboard the Spektr RG (SRG) mission. It was designed to perform the most sensitive all-sky X-ray survey to date, mapping millions of active galactic nuclei and galaxy clusters to study the evolution of the large-scale structure of the Universe and the nature of dark energy.