Properties of Fossil Groups of Galaxies

TL;DR

Fossil groups of galaxies are studied to understand their formation, evolution, and current state, revealing that their large magnitude gap may result from internal processes rather than solely old age.

Contribution

This review synthesizes observational and theoretical insights, challenging the traditional view of fossil groups as exclusively old, relaxed systems.

Findings

Fossil groups may be transitional rather than purely old systems.

Observations question the idea that fossil groups are solely formed by early mergers.

Internal processes could play a significant role in the large magnitude gap formation.

Abstract

We review the formation and evolution of fossil groups and clusters from both the theoretical and the observational points of view. In the optical band, these systems are dominated by the light of the central galaxy. They were interpreted as old systems that had enough time to merge all the M* galaxies within the central one. During the last two decades many observational studies were performed to prove the old and relaxed state of fossil systems. The majority of these studies, that spans a wide range of topics including halos global scaling relations, dynamical substructures, stellar populations, and galaxy luminosity functions, seem to challenge this scenario. The general picture that can be obtained by reviewing all the observational works is that the fossil state could be transitional. Indeed, the formation of the large magnitude gap observed in fossil systems could be related to internal processes rather than an old formation.