Creating more intricate maps of the cosmic web
Our universe is like a vast, three-dimensional spiderweb, made of a complex network of clustered galaxies connected by threads of gas and dark matter known as filaments. Scientists can trace this ‘cosmic web’ through optical galaxy surveys, which reveal filaments, and X-ray data that uncover the hot gas emanating from clusters. While our understanding of the cosmic web has grown significantly in recent decades, there is much more to learn. In the MEMORY project, which was supported by the Marie Skłodowska-Curie Actions(opens in new window) (MSCA) programme, researchers explored how matter moves and evolves through the cosmic web, drawing on X-ray data from the eROSITA telescope(opens in new window). “We focused on galaxy clusters, which grow by pulling in matter along the filaments,” says Nicola Malavasi, a former MSCA fellow and principal investigator in the MEMORY project, now a postdoctoral researcher at the Max Planck Institute for Extraterrestrial Physics(opens in new window). “Because of this, we suspected that the number of filaments connected to a cluster could be closely linked to its mass,” he adds. The team set out to test how strong this relationship is and whether it also holds for two extreme types of structures: lower-mass galaxy groups and the largest structures in the universe, called ‘superclusters’, giant assemblies of galaxy clusters.
Using X-ray data to study superclusters
The work began with the difficult task of detecting the cosmic web itself, using the distribution of galaxies as tracers of its underlying structure. This allowed the researchers to map filaments across a large volume of the universe, which they then used to count how many filaments are connected to galaxy clusters detected by eROSITA. “The analysis was particularly challenging because the eROSITA cluster catalogue covers roughly half the sky,” explains Esra Bulbul, associate professor at the Max Planck Institute for Extraterrestrial Physics. The team then constructed a relationship between a cluster’s mass and the number of filaments linked to it. For superclusters, the researchers focused on a smaller region of the sky where the Sloan Digital Sky Survey(opens in new window) overlaps with the eROSITA observations. They compared their new filament maps with an existing catalogue of superclusters, and used eROSITA data to investigate gas spread between galaxy clusters belonging to the same supercluster.
Discovering a link between cluster mass and filaments
One major discovery was a clear connection between the mass of galaxy clusters and the number of filaments attached to them. This work is currently being prepared for publication. “Our analysis revealed that the main limitation is no longer the number of galaxy clusters available, as in earlier studies, but the quality of the galaxy distance measurements used to reconstruct the cosmic web,” remarks Malavasi. This analysis would ideally use highly precise spectroscopic observations, but as no survey matches the extent of the eROSITA dataset, the team relied on less precise photometric red shifts (measurements that show how far away galaxies are). “Our results, therefore, highlight the need for new large-scale surveys with more accurate galaxy red-shift measurements to push this field beyond the current state of the art,” notes Bulbul.
Advancing our understanding of the cosmic web
In the future, the team hopes to exploit new spectroscopic surveys to improve the reconstruction of the cosmic web and filaments. “In parallel, we aim to extend this work to higher red shifts, allowing us to study how the cosmic web and its connectivity evolve further back in cosmic time,” says Malavasi.
