Post-processing of galaxies due to major cluster mergers

I. Hints from galaxy colours and morphologies

¹ Universidad Técnica Federico Santa María, Av. España 1680, Valparaíso, Chile
² Instituto de Física y Astronomia, Universidad de Valparaíso, 1111 Gran Bretaña, Valparaíso, Chile
e-mail: kkshitija.astro@gmail.com
³ Instituto de Radioastronomia y Astrofisica, UNAM, Campus Morelia, Michoacán, CP 58089, Mexico
⁴ European Southern Observatory (ESO), Alonso de Cordova 3107, Santiago, Chile
⁵ INAF-Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy

Received: 4 August 2023
Accepted: 22 September 2023

Abstract

The environments of galaxy clusters that underwent a recent (≤3 Gyr) major merger are harsher than those of dynamically relaxed clusters due to the global hydrodynamical disturbance and the merger-shock-heated intracluster medium. However, the impact of such extreme cluster interactions on the member galaxy properties is not very well constrained. We explore the integrated star formation properties of galaxies through galaxy colours as well as the morphology buildup in three nearby (0.04 <  z <  0.07) young (∼0.6−1 Gyr) post-merger clusters – A3667, A3376, and A168 – and seven relaxed clusters to disentangle merger-induced post-processing signatures from the expected effects of high-density cluster environments. Exploiting optical spectroscopy and photometry from the OmegaWINGS survey, we find that post-merger clusters are evolved systems with uniform spiral fractions, a uniform fraction of blue galaxies, and constant scatter in the colour–magnitude relations, a regularity that is absent in dynamically relaxed clusters. While no clear merger-induced signatures were revealed in the global colours of galaxies, we conclude that different global star formation histories of dynamically relaxed clusters lead to considerable scatter in galaxy properties, resulting in the pre-merger cluster environment potentially contaminating any merger-induced signal of galaxy properties. We find red spirals to be common in both post-merger and relaxed clusters, while post-merger clusters appear to host a non-negligible population of blue early-type galaxies. We propose that while such merging cluster systems absorb extra cosmic web populations hitherto not part of the original merging subclusters, a ∼1 Gyr timescale is possibly too short see changes in the global colours and morphologies of galaxies.