universe

Fossil groups (FGs) were initially believed to represent the end stage of galaxy group evolution, characterised by a significant luminosity gap between their two brightest member galaxies which was seen as an indicator of their advanced age. In this picture, FGs were thought to undergo a long passive evolution in which their bright satellites gradually merged into the central galaxy, making them fossil relics of the ancient Universe. However, results from the Fossil Group Origins (FOGO) project suggest a paradigm shift: FGs are not necessarily old, but rather isolated systems whose location within the cosmic web primarily regulates their evolution. In this scenario, the magnitude gap arises at low redshift thanks to the infall of galaxies on radial orbits, which enhances the merging rate observed in FGs.

In this seminar, I will review the progress achieved over the last 15 years that led to this new interpretation. I will discuss the luminosity functions, substructures, cosmic web environment, and orbital distribution of FG galaxies, also highlighting the view of FGs as transitional systems. Numerical simulations will also be presented to support the observational evidence, reinforcing the idea that the magnitude gap is a relatively recent feature rather than the imprint of an early formation epoch.