Fossil group origins XIII. A paradigm shift: fossil groups as isolated structures rather than relics of the ancient Universe

TL;DR

This study challenges the traditional view of fossil groups as ancient relics, showing they are more isolated from the cosmic web and likely formed through merging processes due to their unique positions.

Contribution

It provides observational evidence that fossil groups are isolated structures, not necessarily old relics, and introduces a new paradigm for their formation based on cosmic web interactions.

Findings

Fossil systems are more isolated from filaments and intersections than non-fossil systems.

No correlation between magnitude gap size and cosmic web distance.

Fossil systems are located in regions less influenced by cosmic web intersections.

Abstract

In this work we study the large-scale structure around a sample of non-fossil systems and compare the results with earlier findings for a sample of genuine fossil systems selected using their magnitude gap. We compute the distance from each system to the closest filament and intersection as obtained from a catalogue of galaxies in the redshift range $0.05 \le z \le 0.7$. We then estimate the average distances and distributions of cumulative distances to filaments and intersections for different bins of magnitude gap. We find that the average distance to filaments is $(3.0\pm 0.8)$ $R_{200}$ for fossil systems, whereas it is $(1.1\pm 0.1)\,R_{200}$ for non-fossil systems. Similarly, the average distance to intersections is larger in fossil than in non-fossil systems, with values of $(16.3\pm 3.2)$ and $(8.9\pm 1.1) \,R_{200}$, respectively. Moreover, the cumulative distributions of distances to intersections are statistically different between fossil and non-fossil systems. Fossil systems selected using the magnitude gap appear to be, on average, more isolated from the cosmic web than non-fossil systems. No dependence is found on the magnitude gap (i.e. non-fossil systems behave in a similar manner independently of their magnitude gap and only fossils are found at larger average distances from the cosmic web). This result supports a formation scenario for fossil systems in which the lack of infalling galaxies from the cosmic web, due to their peculiar position, favours the building of the magnitude gap via the merging of all the massive satellites with the central galaxy. Comparison with numerical simulations suggests that fossil systems selected using the magnitude gap are not old fossils of the ancient Universe, but systems located in regions of the cosmic web not influenced by the presence of intersections.