An international team of astronomers, led by Andra Stroe from the Harvard–Smithsonian Center for Astrophysics (CfA), has made significant strides in understanding the complex merging process of a nearby low-mass galaxy cluster designated PSZ2 G181.06+48.47. Utilizing NASA’s Chandra and the European Space Agency’s (ESA) XMM-Newton X-ray observatories, the team performed multiwavelength observations that shed light on this galaxy cluster’s intricate properties. The findings were published on January 13, 2025, on the pre-print server arXiv, offering valuable insights into its structure, thermodynamics, and ongoing merger.
What is a Galaxy Cluster and Why Study Them?
Galaxy clusters are massive assemblies of galaxies bound together by gravity. These collections of galaxies contain hundreds, even thousands, of individual galaxies, alongside vast amounts of gas and dark matter. Galaxy clusters are the largest known gravitationally bound structures in the universe and play a crucial role in our understanding of cosmology and the evolution of galaxies.
These colossal structures are of particular interest to astronomers because they serve as natural laboratories for studying various phenomena like galaxy formation, the effects of dark matter, and the evolution of large-scale cosmic structures. Clusters also provide vital clues to the way different physical processes shape the universe.
PSZ2 G181.06+48.47: A Low-Mass Galaxy Cluster with a Complex Structure
The galaxy cluster PSZ2 G181.06+48.47 is located at a redshift of 0.24, corresponding to a distance of about 3.5 billion light-years from Earth. Despite being classified as a low-mass cluster, it holds an intriguing assortment of characteristics, including massive radio structures. The cluster has a mass of approximately 257 trillion solar masses, which is considerably lower than previously estimated, yet still allows it to host two gigantic radio relics — vast, elongated regions of synchrotron radio emission, believed to originate from the interactions of energetic particles in the cluster’s magnetic fields.
This galaxy cluster spans an impressive radius of around 3.45 million light-years, and its radio relics extend over 3.9 million light-years, revealing the cluster’s history of dynamic and violent processes. Additionally, PSZ2 G181.06+48.47 hosts a candidate radio halo, a diffuse emission of low-frequency radio waves that covers a large portion of the cluster, demonstrating a highly energetic environment.
A Merging Cluster: Key Clue Revealed
Although the discovery of these giant radio structures was intriguing, astronomers still lacked clear evidence about the merger dynamics within PSZ2 G181.06+48.47. Galaxy cluster mergers often exhibit distinct physical signatures in various wavelengths — including X-ray emissions from the hot, ionized gas between galaxies, which traces the dynamics of cluster collisions.
The X-ray observatories, Chandra and XMM-Newton, enabled a deeper look into the cluster’s interior. What the team discovered was compelling evidence that PSZ2 G181.06+48.47 is undergoing a merger between two subclusters. This ongoing process is crucial in understanding how galaxy clusters evolve and how substructures like these influence large-scale cosmic dynamics.
The analysis confirmed that PSZ2 G181.06+48.47’s X-ray morphology is elongated along a northeast-southwest axis, a clear indicator of a merger process. The cluster displays two distinct X-ray peaks separated by about 1.2 million light-years, which are connected through a bridge of X-ray emission. This configuration is typical of post-core passage mergers, where two subclusters have passed through each other and are now in the process of settling into a combined gravitational equilibrium.
Thermodynamic and Morphological Features of PSZ2 G181.06+48.47
One of the key findings in this study is that PSZ2 G181.06+48.47 is a cooler galaxy cluster compared to others previously studied with XMM-Newton. The average temperature in the cluster’s global emission was determined to be around 3.86 keV, suggesting a relatively lower temperature for a galaxy cluster, which could indicate either its developmental stage or the lower thermal energy of its intergalactic medium.
Temperature variations within the cluster were found, too. The researchers noted a temperature gradient, with the northern core region exhibiting temperatures around 3.2 keV, rising to about 5.5 keV in the southern core of the cluster. This increase in temperature is likely associated with dynamic shock fronts created by the ongoing merger process. The lower temperature of the cluster as a whole suggests that PSZ2 G181.06+48.47 is in an intermediate stage of development, still undergoing substantial energetic interactions between its constituent subclusters.
Additionally, the X-ray emission of the cluster is consistent with a system in which the two subclusters are on the verge of merging but have not yet fully come together, showing significant signs of post-merger activity. This could indicate a late-stage merger, where the two subclusters are observed after they’ve passed through one another, continuing to interact gravitationally as they gradually settle.
Radio Relics: The Broadest Separation Ever Seen
The study’s findings on radio relics in PSZ2 G181.06+48.47 are particularly remarkable. The radio relics in this cluster, which span an enormous distance of 3.9 million light-years, have the widest separation of all known double-relic clusters. These relics are critical in understanding the nature of the collision between the two subclusters, as the shocks created during a merger can accelerate charged particles and produce the synchrotron radio emission seen in the relics.
Radio relics provide crucial information about the nature of cosmic shocks and the role of magnetic fields in the large-scale structures of the universe. In PSZ2 G181.06+48.47, the existence of such widespread radio relics is indicative of high-energy, shock-driven processes. The team behind this study concluded that these radio relics most likely formed as a result of energetic particle acceleration due to a major shock collision between the merging subclusters.
Conclusion: A Complex Picture of a Merging Galaxy Cluster
The detailed observations provided by Chandra and XMM-Newton have painted a clearer picture of PSZ2 G181.06+48.47, a merging galaxy cluster in the late stages of interaction. With a mass of 257 trillion solar masses, this cluster is considered one of the smallest known to host double radio relics, which is an intriguing anomaly in cluster studies.
From an X-ray perspective, the structure of the cluster suggests a highly disturbed system, marked by the complex thermodynamic distribution of gas and the clearly defined substructure. The findings support the scenario that PSZ2 G181.06+48.47 is a post-merger system — where the two subclusters are falling back toward each other after crossing each other’s cores. This unique view offers new insights into the evolution of galaxy clusters and their complex merger dynamics.
Reference: Andra Stroe et al, PSZ2 G181.06+48.47 I: X-ray exploration of a low-mass cluster with exceptionally-distant radio relics, arXiv (2025). DOI: 10.48550/arxiv.2501.07651