The Fossil Phase in the Life of a Galaxy Group

TL;DR

This study uses cosmological simulations to explore the formation and evolution of fossil galaxy groups, highlighting their early formation, high dark matter concentration, and the role of satellite mergers in creating large magnitude gaps.

Contribution

It provides new insights into the physical mechanisms behind fossil group formation, emphasizing early infall and merger processes in a cosmological context.

Findings

Fossil groups have high dark matter concentrations linked to early formation.

Large magnitude gaps form after groups accumulate half their mass due to mergers.

Fossil systems are primarily driven by early infall of massive satellites.

Abstract

We investigate the origin and evolution of fossil groups in a concordance LCDM cosmological simulation. We consider haloes with masses between $(1-5)\times10^{13} \hMsun$ and study the physical mechanisms that lead to the formation of the large gap in magnitude between the brightest and the second most bright group member, which is typical for these fossil systems. Fossil groups are found to have high dark matter concentrations, which we can relate to their early formation time. The large magnitude-gaps arise after the groups have build up half of their final mass, due to merging of massive group members. We show that the existence of fossil systems is primarily driven by the relatively early infall of massive satellites, and that we do not find a strong environmental dependence for these systems. In addition, we find tentative evidence for fossil group satellites falling in on orbits with typically lower angular momentum, which might lead to a more efficient merger onto the host. We find a population of groups at higher redshifts that go through a ``fossil phase'': a stage where they show a large magnitude-gap, which is terminated by renewed infall from their environment.