Star formation as a function of environment in the MeerKAT Galaxy Clusters Legacy Survey.

Abstract

Probing the star formation (SF) activity of cluster galaxies paves an important path towards the understanding of cluster evolution. This thesis presents the study of star formation rates (SFR) in clusters using dustunbiased radio luminosities from the MeerKAT Galaxy Clusters Legacy Survey (MGCLS). Our radio data is complemented by optical data from the Dark Energy Camera Legacy Survey (DECaLS), for photometric redshifts, and also Sunyaev-Zel’dovich (SZ) effect-derived cluster masses from the Atacama Cosmology Telescope (ACT). We present the first statistical study of SFR in clusters using MeerKAT-detected galaxies which takes advantage of MeerKAT’s large field of view to investigate the relation between SF activity and cluster environments out to 2R200. Using radio diffuse emission in the form of haloes and relics as a proxy for cluster merger activity, we divide our cluster sample between disturbed/merger clusters and relaxed clusters. We observe a higher fraction of star-forming galaxies (fSF) in disturbed clusters than in relaxed clusters. Disturbed clusters also have higher masses (M200) and total SFR (ΣSFR) in contrast to relaxed clusters. On analysing the redshift evolution of the massnormalised ΣSFR, we observe a ≈ 4× decline in the SF activity of clusters from the redshift of 0.35 to 0.15, corresponding to ≈ 2 Gyr in look-back time. Our result is roughly consistent with the one from cluster studies that used infrared-derived SFR (≈ 5× decline) at a similar redshift slice as our sample. We use a subsample of double relic-hosting clusters to investigate the relation between cluster SF activity and the time that has passed since the merger started (tmerger) estimated from the relic distances from cluster cores. We observe an anti-correlation between ΣSFR and tmerger, suggesting that younger mergers have a higher SF activity. However, we see no clear correlation in the mass-normalised ΣSFR with tmerger. We also investigate for differences in the SF activity of galaxies closer to radio relics and those away from the relics and observe no significant differences between the two populations.