# The dynamic stage of clusters and its influence on the stellar populations of galaxies

## TL;DR

This study examines how the dynamical state of galaxy clusters affects the stellar populations of their member galaxies, revealing that unrelaxed clusters host younger stars, especially in low-mass galaxies on the outskirts.

## Contribution

It introduces a new kinematic substructure detection code and analyzes the impact of cluster relaxation on galaxy stellar ages using SDSS spectra.

## Key findings

- Unrelaxed clusters have younger stellar populations than relaxed ones.
- Secondary subhalos in unrelaxed clusters contain predominantly younger, low-mass galaxies.
- An age-radius relation exists for all subhalos regardless of substructure presence.

## Abstract

We investigate the stellar populations of galaxies in clusters at different dynamical stages, aiming to identify possible effects of the relaxation state of the cluster or subcluster on the star formation histories of its galaxies. We have developed and applied a code for kinematic substructure detection to a sample of 412 galaxy clusters drawn from the \citet[]{tempel2012} catalogue, finding a frequency of substructures of 45\%. We have extracted mean stellar ages with the {\sc{starlight}} spectral synthesis code applied to SDSS-III spectra of the sample galaxies. We found lower mean stellar ages in unrelaxed clusters relative to relaxed clusters. For unrelaxed clusters, we separated primary and secondary subhalos and found that, while relaxed clusters and primaries present similar masses and age distributions, secondaries present younger stellar populations, mainly due to low-mass galaxies ($\log M_\star/M_\odot \lesssim 11$\,dex). An age-clustercentric radius relation is seen for all subhalos irrespective of the presence of substructures. We also observe relations between the mean stellar age and mass of relaxed and unrelaxed clusters, massive systems presenting higher mean ages. The locus of these relations is distinct between relaxed and unrelaxed clusters, but become indistinguishable when separating primaries and secondaries. Our results suggest that differences between relaxed and unrelaxed clusters are mainly driven by low-mass systems in the clusters outskirts, and that, while pre-processing can be seen in the subcomponents of dynamically young clusters, some evolution in the stellar populations must occur during the clusters relaxation.