The large-scale structure of the Universe could be divided into filaments, nodes and walls, what we call the cosmic web. A large web ‘woven’ with threads of dark matter that would bind together what is considered ordinary matter. Galaxies evolve within this web and the question, still open, is the impact it has on this process. A paper by CEFCA researcher Stefano Zarattini points to the role that filaments could play. The research suggests that the difference that would determine whether a group of galaxies is gas-poor or gas-rich would be its position relative to a filament. The position of the clusters will determine the influence that filaments exert on those clusters. Filaments resist the outward flow of gas. Thus, they become a mechanism to control the outward flow of gas.

The result is that clusters near the filament centre will have a higher fraction of gas near their core and thus a higher surface brightness seen in X-rays. Consequently, the distance to the filaments would be larger for clusters with low X-ray surface brightness. The projected distance between each cluster and the spine of the nearest filament would therefore be a key element in understanding the influence of large-scale structure on galaxy evolution.

This behaviour, which has already been observed for nodes, would, according to this paper, establish that the position of the clusters within the network would have a direct impact on their gas mass. The filament would act as a kind of ‘cage’ that traps the gas and allows it to return more quickly to the central regions of the cluster. The study was constructed from 29 of the 34 clusters in the unbiased X-ray sample called XUCS.

The research is published in Astronomy & Astrophysics. It is signed by Stefano Zarattini, from the Centre for the Study of the Physics of the Cosmos (CEFCA) and, together with him, Stefano Andreon and Emanuella Puddu, both from the Istituto Nazionale di Astrofisica in Italy. All three agree that their field of study is to understand how galaxies and galaxy clusters evolve.

The trend is that gas-poor systems are found at greater distances than gas-rich ones and, therefore, the authors argue that the position of the clusters would affect the mechanisms that determine their gas reservoir. Gas propelled to great distances by the active galactic nucleus (AGN) would encounter resistance from the filament and thus reduce the distance and velocity it can reach. The return would also be faster, so that both processes would make the clusters close to the backbone of a filament richer in gas.

These mechanisms are key to understanding the factors that modulate gas in galaxies. Zarattini's research suggests that gas-poor clusters are, on average, found at greater distances from gas-rich filaments. The next necessary step would be to study a larger sample of clusters to reinforce the conclusions of this research, which opens the way to understanding the role of the so-called cosmic web in the evolution of galaxies.