Ultimate age-dating method for galaxy groups; clues from the Millennium Simulations

TL;DR

This paper proposes a new method combining four observable parameters to more accurately identify old galaxy groups in simulations, improving upon the traditional luminosity gap criterion.

Contribution

It introduces a multi-parameter approach for age-dating galaxy groups, reducing contamination from non-old groups compared to using luminosity gap alone.

Findings

A set of four observational parameters effectively identifies old galaxy groups.

Combining parameters significantly improves age-dating accuracy.

Luminosity gap alone is insufficient for reliable age classification.

Abstract

There have been a number of studies dedicated to identification of fossil galaxy groups, arguably groups with a relatively old formation epoch. Most of such studies identify fossil groups, primarily based on a large luminosity gap, which is the magnitude gap between the two most luminous galaxies in the group. Studies of these types of groups in the millennium cosmological simulations show that, although they have accumulated a significant fraction of their mass, relatively earlier than groups with a small luminosity gap, this parameter alone is not highly efficient in fully discriminating between the "old" and "young" galaxy groups, a label assigned based on halo mass accumulation history. We study galaxies drawn from the semi-analytic models of Guo et al. (2011), based on the Millennium Simulation. We establish a set of four observationally measurable parameters which can be used in combination, to identify a subset of galaxy groups which are old, with a very high probability. We thus argue that a sample of fossil groups selected based on luminosity gap will result in a contaminated sample of old galaxy groups. By adding constraints on the luminosity of the brightest galaxy, and its offset from the group luminosity centroid, we can considerably improve the age-dating.