Star formation as a function of environment in the MeerKAT Galaxy Clusters Legacy Survey.
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Date
2023
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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.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.