Merger relics of cluster galaxies

TL;DR

This paper develops a theoretical framework using simulations to explain the high observed fraction of merger features in massive cluster galaxies, suggesting these features are relics from past mergers rather than recent events.

Contribution

It introduces a combined simulation approach to estimate merger feature lifetimes and explains the persistence of merger signs in cluster galaxies as relics from earlier halo environments.

Findings

Merger features can last very long in cluster environments.

Observed merger features may be relics from previous halo mergers.

Simulation results closely match observed merger feature fractions.

Abstract

Context. Sheen and collaborators recently found that a surprisingly large portion (38%) of massive early-type galaxies in heavy clusters show strong merger-related disturbed features. This contradicts the general understanding that massive clusters are hostile environments for galaxy mergers. Considering the significance of mergers in galaxy evolution, it is important to understand this. Aims. We aim to present a theoretical foundation that explains galaxy mergers in massive clusters. Methods. We used the N-body simulation technique to perform a cosmological-volume simulation and derive dark-halo merger trees. Then, we used the semi-analytic modeling technique to populate each halo with galaxies. We ran hydrodynamic simulations of galaxy mergers to estimate the lifetime of merger features for the imaging condition used by Sheen and collaborators. We applied this merger feature lifetime to our semi-analytic models. Finally, we counted the massive early-type galaxies in heavy model clusters that would show strong merger features. Results. While there still are substantial uncertainties, our preliminary results are remarkably close to the observed fraction of galaxies with merger features. Key ingredients for the success are twofold: firstly, the subhalo motion in dark haloes has been accurately traced, and, second, the lifetime of merger features has been properly estimated. As a result, merger features are expected to last very long in cluster environments. Many massive early-type galaxies in heavy clusters therefore show merger features not because they experience mergers in the current clusters in situ, but because they still carry their merger features from their previous halo environments. Conclusions. Investigating the merger relics of cluster galaxies is potentially important, because it uniquely allows us to backtrack the halo merger history.