Diffuse radio emission in ACTPol clusters.
Date
2021
Journal Title
Journal ISSN
Volume Title
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Abstract
Low-frequency radio observations of galaxy clusters reveal cluster-scale diffuse emission that
is not associated with individual galaxies. Studying the properties of these diffuse radio sources
gives insight into astrophysical processes such as cosmic ray transportation in the intracluster
medium (ICM). Observations have linked the formation of radio halos and relics with turbulence
caused by cluster mergers and the formation of mini-halos to gas sloshing in cool-core clusters.
Statistical studies of large galaxy cluster samples have been used to determine how the radio
properties of diffuse emission scale with the mass and X-ray luminosity of the host clusters.
Such studies are crucial for refining the formation theories of diffuse emission. New generation
telescopes with wide bandwidths and high sensitivity such as the upgraded Giant Metrewave Radio
Telescope (uGMRT) andMeerKAT are advantageous for the study of faint extended emission
in large cluster samples.
The main aim of this thesis was to do an in-depth study of the diffuse radio emission using
a cluster sample that spans a wider mass and redshift range compared to the currently studied
parameter space. We developed data reduction techniques for calibrating data from telescopes
such as uGMRT and MeerKAT. The wide bandwidth of these telescopes introduces directional
dependent effects (DDEs) that make the calibration process extremely complicated. However,
such observations are excellent for studies of the faint diffuse emission and in-band spectral
indices of this emission.
In the first part of this thesis, we focused on the study of diffuse radio emission in a Sunyaev-
Zeldovich (SZ) selected sample of clusters. These clusters were observed by the Atacama Cosmology
Telescope’s Polarimetric extension (ACTPol). We used archival and new GMRT observations
for the radio analysis of this sample. We reported newly detected diffuse emission in the
following clusters: a radio halo and revived fossil plasma in ACT-CL J0137.4 0827, a radio relic
in ACT-CL J2128.4+0135, and a candidate relic in ACT-CL J0022.2 0036. The radio analysis
of the full sample revealed that the fraction of clusters in the sample hosting diffuse emission
is 26.7% excluding candidate emission and 30% when it is included. The detection rate of the
diffuse emission over all categories is lower than the detection rates reported in literature. We
note that this may be because the sample comprised high redshift (z ¡ 0.5) and low mass clusters
(M500c;SZ 5 1014 Md), though future more sensitive observations of these clusters could reveal
fainter diffuse emission structures. We compared our results to the most recent radio halo and radio
relic scaling relations. The radio halo P1:4GHz M500 scaling relation plot indicates that a few
flatter spectrum radio halos are located in the region previously known to be populated by ultrasteep
spectrum radio halos (USSRHs). Finally, we presented preliminary results of the uGMRT
wideband backend (GWB) data reduction for ACT-CL J0034.4+0225, ACT-CL J0137.4 0827,
and ACT-CL J2128.4+0135. We prioritised these clusters because the narrowband data revealed
that they host diffuse emission. However, once the data reduction algorithm is improved, we
will reduce the remaining clusters with non-detections. Comparing the GWB results to the narrowband
GMRT data, we note that the radio halo observed in ACT-CL J0137.4 0827 is more
extended in the GWB data. The diffuse emission is detected at a higher signal-to-noise ratio in
the GWB images for the three clusters. We note that an improvement in the GWB reduction
algorithm might reveal diffuse emission that was not detected in the narrowband data.
In the second part of the thesis, we used MeerKAT observations to study diffuse emission in
the Bullet Cluster (1E0657 56), RXCJ1314.4 2515, Abell 3562, and Abell 3558. We detected
new extended features in the radio halos hosted by the Bullet cluster and Abell 3562. We assume
that the decrement feature in the Bullet cluster might be an indication of a second wave of merger
activity. The ridge feature in the peripheral region of the radio halo in Abell 3562 overlaps with
the edge of the X-ray emission. Hence, we assume that the feature might be related to a shock
region. We also reported the detection of a new mini-halo in Abell 3558. MeerKAT’s sensitivity
and wide bandwidth enabled us to perform in-band spectral index studies and produce spectral
index maps for the Bullet cluster, RXCJ1314.4 2515, and Abell 3562. The spectral index maps
of the relics in the Bullet cluster and RXCJ1314.4 2515 indicate a spectral steepening towards
the cluster center, while the spectral index map of the radio halo in the Bullet cluster indicates
radial spectral steepening. The spectral index map of Abell 3562 indicates that the radio halo
and ridge have similar spectral index variations, which suggests that the ridge feature is related
to the radio halo.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.