A GMRT Narrowband vs. Wideband Analysis of the ACT-CL J0034.40225 Field Selected from the ACTPol Cluster Sample+

Low frequency radio observations of galaxy clusters are a useful probe of the non-thermal intracluster medium (ICM), through observations of diffuse radio emission such as radio halos and relics. Current formation theories cannot fully account for some of the observed properties of this emission. In this study, we focus on the development of interferometric techniques for extracting extended, faint diffuse emissions in the presence of bright, compact sources in wide-field and broadband continuum imaging data. We aim to apply these techniques to the study of radio halos, relics and radio mini-halos using a uniformly selected and complete sample of galaxy clusters selected via the Sunyaev-Zel’dovich (SZ) effect by the Atacama Cosmology Telescope (ACT) project, and its polarimetric extension (ACTPol). We use the upgraded Giant Metrewave Radio Telescope (uGMRT) for targeted radio observations of a sample of 40 clusters. We present an overview of our sample, confirm the detection of a radio halo in ACT−CL J0034.4+0225, and compare the narrowband and wideband analysis results for this cluster. Due to the complexity of the ACT−CL J0034.4+0225 field, we use three pipelines to process the wideband data. We conclude that the experimental spam wideband pipeline produces the best results for this particular field. However, due to the severe artefacts in the field, further analysis is required to improve the image quality.

Exploring Diffuse Radio Emission in the Southern Sky

<p>This thesis investigates currently observed correlations between the thermal and non-thermal (radio halos) components of galaxy clusters, and seeks to verify the reliability of the proposed radio halo scaling relations presented in the literature. It employs a two-pronged approach: 1) a statistical examination of 15 galaxy clusters; and 2) detailed multi-wavelength analysis of individual objects of interest. We first investigated radio data for 15 galaxy clusters drawn from the parent REXCESS sample observed with the ATCA at 1.4 GHz to conduct a radio halo survey. Examination of available and re-processed low resolution images revealed cluster-scale diffuse objects in three clusters. One was a radio halo candidate in Abell 3888 (A3888), with the two remaining diffuse sources being radio relic candidates. Follow-up observations of the candidate clusters were performed in July and December 2011, and March 2012, with the upgraded ATCA (CABB). Radio observations with CABB in different array configurations were used to provide the required resolution and sensitivity to a wider range of angular scales to probe the candidate diffuse sources. Examination of the final CABB images confirmed the existence of the radio halo in A3888; however, the remaining candidates were found to be a head-tail galaxy and a very bright radio galaxy with extended emission. As this thesis presents some of the earliest CABB observations, new data reduction and imaging procedures were necessarily developed and presented here. The statistical component of this thesis uses a halo sample obtained from the combined detection of this work and the literature to derive new correlations between the cluster observables and the radio halo power. The new correlation between the X-ray luminosity and radio halo power derived here is flatter than the previous correlation in the literature, suggesting that massive clusters may host lower power halos than previously thought. In addition, we derived the upper limits of the undetected power of possible radio halos for our non-halo clusters via injection of fake radio halos into the UV data. Our derived upper limits with respect to the anticipated halo powers according to the previous and new correlations and their interpretations are discussed in the thesis. The distribution of the combined upper limits (this work and the literature) compared to our new correlation shows no sign of the strong bi-modality found in the literature. As previously mentioned, we detected a giant radio halo in A3888. We observed A3888 with the AAOmega spectrograph to infer the dynamics of the cluster. We measured the spectra of 254 galaxies within a 300 radius from the core of A3888 and combined these data with the available literature redshifts in the region. We identified 71 member galaxies of A3888 and examined the density contours, which indicated that the distribution of the member galaxies is elongated along an east-west axis. This elongation might be indicative of dynamical interactions in the cluster; however, there is no statistically significant deviation from Gaussianity in the velocity data. We then carried out a Lee-Fitchett 3D substructure test and found that A3888 is bimodal and has two subgroups. The head-tail galaxy mentioned earlier in one of the clusters was originally thought to be a radio relic candidate. Owing to available broadband polarimetric data and well-separated jets of the head-tail galaxy, we investigated the magnetic field direction of the head-tail galaxy and conclude it is highly likely that a helical magnetic field is present in the jets. We present the high-resolution images of our 15 clusters and create a catalogue of the detected sources. Finally, we discuss concerns with the current radio halo detections in the literature, and how radio halo surveys could be designed in the future to yield unbiased results.</p>

A Radio View of the Bullet Cluster from 100 MHz to 9 GHz

<p>We investigate a sample of 10 massive galaxy clusters for diffuse synchrotron emission. The shortlisted clusters are drawn from a sample of clusters observed with the South Pole Telescope (SPT) shown to have high Sunyaev-Zeldovich (SZ) signals.They are analysed for diffuse emission from the results of the Australia Telescope Compact Array (ATCA) archival data reduction. The focus then is on the cluster with the most prominent diffuse emission - the Bullet cluster. We used the Murchison Widefield Array Commissioning Survey (MWACS) data in conjunction with the ATCA images to derive the spectral behaviour of the Bullet cluster from 0.118 GHz to 8.896 GHz. In particular, we study the spectral properties of the known radio halo and radio relic. We search for spectral bending of this diffuse emission as seen in other clusters like the Coma cluster, A2256, A521 and A3256. We detect the radio relic at all frequencies in the cluster periphery. Polarised flux is detected for the relic at all frequencies except at 1.344 GHz and as expected the percentage polarisation increases with frequency. Our spectral index values of -1.08 ± 0.02 and -1.74 ± 0.22 for 2 regions of the radio relic agreed with the literature. We detect spectral flattening for a region in the radio relic at 4.532 GHz. This is a common spectral characteristic for a radio galaxy. This suggests that the source could be a recently dead radio galaxy. We discuss a scenario in which a dead radio galaxy supplying seed electrons for reacceleration and a merger process providing the required energy for the diffuse radio relic. We detect the radio halo at all frequencies and we derive a spectral index of -2.11±0.03 using our ATCA flux measurements. Our individual flux measurements at 1.344 and 2.1 GHz agree with the literature. However, we get a steeper ATCA spectral index value for the radio halo as compared to the existing value in the literature. We observe spectral flattening of the radio halo in the Bullet cluster at low frequencies between 0.180 GHz and 1.3 GHz. This is similar to the spectral property of the halo in clusters like the Coma cluster, A521 and A3256.</p>

Exploring Diffuse Radio Emission in the Southern Sky

<p>This thesis investigates currently observed correlations between the thermal and non-thermal (radio halos) components of galaxy clusters, and seeks to verify the reliability of the proposed radio halo scaling relations presented in the literature. It employs a two-pronged approach: 1) a statistical examination of 15 galaxy clusters; and 2) detailed multi-wavelength analysis of individual objects of interest. We first investigated radio data for 15 galaxy clusters drawn from the parent REXCESS sample observed with the ATCA at 1.4 GHz to conduct a radio halo survey. Examination of available and re-processed low resolution images revealed cluster-scale diffuse objects in three clusters. One was a radio halo candidate in Abell 3888 (A3888), with the two remaining diffuse sources being radio relic candidates. Follow-up observations of the candidate clusters were performed in July and December 2011, and March 2012, with the upgraded ATCA (CABB). Radio observations with CABB in different array configurations were used to provide the required resolution and sensitivity to a wider range of angular scales to probe the candidate diffuse sources. Examination of the final CABB images confirmed the existence of the radio halo in A3888; however, the remaining candidates were found to be a head-tail galaxy and a very bright radio galaxy with extended emission. As this thesis presents some of the earliest CABB observations, new data reduction and imaging procedures were necessarily developed and presented here. The statistical component of this thesis uses a halo sample obtained from the combined detection of this work and the literature to derive new correlations between the cluster observables and the radio halo power. The new correlation between the X-ray luminosity and radio halo power derived here is flatter than the previous correlation in the literature, suggesting that massive clusters may host lower power halos than previously thought. In addition, we derived the upper limits of the undetected power of possible radio halos for our non-halo clusters via injection of fake radio halos into the UV data. Our derived upper limits with respect to the anticipated halo powers according to the previous and new correlations and their interpretations are discussed in the thesis. The distribution of the combined upper limits (this work and the literature) compared to our new correlation shows no sign of the strong bi-modality found in the literature. As previously mentioned, we detected a giant radio halo in A3888. We observed A3888 with the AAOmega spectrograph to infer the dynamics of the cluster. We measured the spectra of 254 galaxies within a 300 radius from the core of A3888 and combined these data with the available literature redshifts in the region. We identified 71 member galaxies of A3888 and examined the density contours, which indicated that the distribution of the member galaxies is elongated along an east-west axis. This elongation might be indicative of dynamical interactions in the cluster; however, there is no statistically significant deviation from Gaussianity in the velocity data. We then carried out a Lee-Fitchett 3D substructure test and found that A3888 is bimodal and has two subgroups. The head-tail galaxy mentioned earlier in one of the clusters was originally thought to be a radio relic candidate. Owing to available broadband polarimetric data and well-separated jets of the head-tail galaxy, we investigated the magnetic field direction of the head-tail galaxy and conclude it is highly likely that a helical magnetic field is present in the jets. We present the high-resolution images of our 15 clusters and create a catalogue of the detected sources. Finally, we discuss concerns with the current radio halo detections in the literature, and how radio halo surveys could be designed in the future to yield unbiased results.</p>

A Radio View of the Bullet Cluster from 100 MHz to 9 GHz

<p>We investigate a sample of 10 massive galaxy clusters for diffuse synchrotron emission. The shortlisted clusters are drawn from a sample of clusters observed with the South Pole Telescope (SPT) shown to have high Sunyaev-Zeldovich (SZ) signals.They are analysed for diffuse emission from the results of the Australia Telescope Compact Array (ATCA) archival data reduction. The focus then is on the cluster with the most prominent diffuse emission - the Bullet cluster. We used the Murchison Widefield Array Commissioning Survey (MWACS) data in conjunction with the ATCA images to derive the spectral behaviour of the Bullet cluster from 0.118 GHz to 8.896 GHz. In particular, we study the spectral properties of the known radio halo and radio relic. We search for spectral bending of this diffuse emission as seen in other clusters like the Coma cluster, A2256, A521 and A3256. We detect the radio relic at all frequencies in the cluster periphery. Polarised flux is detected for the relic at all frequencies except at 1.344 GHz and as expected the percentage polarisation increases with frequency. Our spectral index values of -1.08 ± 0.02 and -1.74 ± 0.22 for 2 regions of the radio relic agreed with the literature. We detect spectral flattening for a region in the radio relic at 4.532 GHz. This is a common spectral characteristic for a radio galaxy. This suggests that the source could be a recently dead radio galaxy. We discuss a scenario in which a dead radio galaxy supplying seed electrons for reacceleration and a merger process providing the required energy for the diffuse radio relic. We detect the radio halo at all frequencies and we derive a spectral index of -2.11±0.03 using our ATCA flux measurements. Our individual flux measurements at 1.344 and 2.1 GHz agree with the literature. However, we get a steeper ATCA spectral index value for the radio halo as compared to the existing value in the literature. We observe spectral flattening of the radio halo in the Bullet cluster at low frequencies between 0.180 GHz and 1.3 GHz. This is similar to the spectral property of the halo in clusters like the Coma cluster, A521 and A3256.</p>

Spectral Index of the Filaments in the Abell 523 Radio Halo

The galaxy cluster Abell 523 hosts a radio halo characterized by the presence of two filaments transversely located with respect to the cluster merger axis. In this paper, we present a spectral index image of these filaments between 1.410 and 1.782 GHz obtained with Jansky Very Large Array observations. We find a steepening of the spectral index of the filaments at frequencies ≳1.4 GHz and an indication that bright patches are characterized by flat spectral indices. Our results are consistent with a scenario of highly-efficient turbulence induced by merger phenomena.

Particle Reacceleration by Turbulence and Radio Constraints on Multimessenger High-energy Emission from the Coma Cluster

Galaxy clusters are considered to be gigantic reservoirs of cosmic rays (CRs). Some of the clusters are found with extended radio emission, which provides evidence for the existence of magnetic fields and CR electrons in the intra-cluster medium. The mechanism of radio halo (RH) emission is still under debate, and it has been believed that turbulent reacceleration plays an important role. In this paper, we study the reacceleration of CR protons and electrons in detail by numerically solving the Fokker–Planck equation, and show how radio and gamma-ray observations can be used to constrain CR distributions and resulting high-energy emission for the Coma cluster. We take into account the radial diffusion of CRs and follow the time evolution of their one-dimensional distribution, by which we investigate the radial profile of the CR injection that is consistent with the observed RH surface brightness. We find that the required injection profile is nontrivial, depending on whether CR electrons have a primary or secondary origin. Although the secondary CR electron scenario predicts larger gamma-ray and neutrino fluxes, it is in tension with the observed RH spectrum for hard injection indexes, α < 2.45. This tension is relaxed if the turbulent diffusion of CRs is much less efficient than the fiducial model, or the reacceleration is more efficient for lower-energy CRs. In both the secondary and primary scenario, we find that galaxy clusters can make a sizable contribution to the all-sky neutrino intensity if the CR energy spectrum is nearly flat.

Diffuse Extragalactic Emission at Low Frequencies within the Epoch of Reionization 0-hour Field

<p>Low-frequency radio imaging of the southern sky has become available with the advent of the Murchison Widefield Array (MWA). The topic of this thesis is the study of extended, low-frequency radio emission, with a primary focus on the non-thermal synchrotron emission associated with the intra-cluster medium (ICM) of galaxy clusters. We do not limit the study to such emission, however, and investigate a small sample of other interesting and extended radio emission from objects in the southern sky. A significant portion of this work is invested in detecting, and characterising, extended, diffuse radio emission from galaxy clusters within a 45 degree by 45 degree region of the southern sky centred on R.A. = 0 hours, decl. = -27 degrees. This field is chosen as a deep MWA image has been made available which is sensitive to extended structures. Within the field we search for low-frequency, diffuse cluster emission, previously detected or otherwise. In doing so we find 34 diffuse radio sources, 3 of which are newly detected haloes, 1 newly detected relic with many new candidates of each. Further, we detect a new phoenix candidate as well as 2 candidate dead radio galaxies at the centre of clusters. We confirm previous observations of such emission as well, and measure properties such as their integrated flux densities, spectral indices, and sizes where possible. We compare our sample of haloes with previously detected haloes and revisit established scaling relations of the radio halo power with the cluster X-ray luminosity and mass. We find that both scaling relations are consistent with previous findings despite the increase in sample size, though note that the raw scatter in the data for best-fitting parameters increases with increase in sample size. In this, we demonstrate the utility of low-frequency radio telescopes like the MWA in detecting such emission, showing that the MWA is pushing into higher-redshift, lower-mass systems, though we caution that the low resolution of the MWA can work against us. We follow-up on two galaxy clusters found to host extended emission - Abell S1136 and Abell S1063. In the case of Abell S1136 we observe the emission at its centre with the Australia Telescope Compact Array (ATCA) and determine the presence of a core, suggesting the emission to be that of an ancient episode of an active galactic nucleus in the central elliptical of the cluster, ESO 470-G020. After reducing archival ATCA data for Abell S1063 we find no evidence of a halo and consider the source to be constructed of blended point sources. We close with a description of a strong double-lobed radio source associated with a non-elliptical host ESO 472-G013, likely a spiral or irregular galaxy, that was found serendipitously whilst searching for diffuse cluster emission. We explore the host within the context of star-formation, and consider the possible origins of the AGN and lobes due to interaction with either the nearby spiral, ESO 472-G012, or a past or ongoing merger event.</p>

Diffuse Extragalactic Emission at Low Frequencies within the Epoch of Reionization 0-hour Field

<p>Low-frequency radio imaging of the southern sky has become available with the advent of the Murchison Widefield Array (MWA). The topic of this thesis is the study of extended, low-frequency radio emission, with a primary focus on the non-thermal synchrotron emission associated with the intra-cluster medium (ICM) of galaxy clusters. We do not limit the study to such emission, however, and investigate a small sample of other interesting and extended radio emission from objects in the southern sky. A significant portion of this work is invested in detecting, and characterising, extended, diffuse radio emission from galaxy clusters within a 45 degree by 45 degree region of the southern sky centred on R.A. = 0 hours, decl. = -27 degrees. This field is chosen as a deep MWA image has been made available which is sensitive to extended structures. Within the field we search for low-frequency, diffuse cluster emission, previously detected or otherwise. In doing so we find 34 diffuse radio sources, 3 of which are newly detected haloes, 1 newly detected relic with many new candidates of each. Further, we detect a new phoenix candidate as well as 2 candidate dead radio galaxies at the centre of clusters. We confirm previous observations of such emission as well, and measure properties such as their integrated flux densities, spectral indices, and sizes where possible. We compare our sample of haloes with previously detected haloes and revisit established scaling relations of the radio halo power with the cluster X-ray luminosity and mass. We find that both scaling relations are consistent with previous findings despite the increase in sample size, though note that the raw scatter in the data for best-fitting parameters increases with increase in sample size. In this, we demonstrate the utility of low-frequency radio telescopes like the MWA in detecting such emission, showing that the MWA is pushing into higher-redshift, lower-mass systems, though we caution that the low resolution of the MWA can work against us. We follow-up on two galaxy clusters found to host extended emission - Abell S1136 and Abell S1063. In the case of Abell S1136 we observe the emission at its centre with the Australia Telescope Compact Array (ATCA) and determine the presence of a core, suggesting the emission to be that of an ancient episode of an active galactic nucleus in the central elliptical of the cluster, ESO 470-G020. After reducing archival ATCA data for Abell S1063 we find no evidence of a halo and consider the source to be constructed of blended point sources. We close with a description of a strong double-lobed radio source associated with a non-elliptical host ESO 472-G013, likely a spiral or irregular galaxy, that was found serendipitously whilst searching for diffuse cluster emission. We explore the host within the context of star-formation, and consider the possible origins of the AGN and lobes due to interaction with either the nearby spiral, ESO 472-G012, or a past or ongoing merger event.</p>

Pilot Study and Early Results of the Cosmic Filaments and Magnetism Survey with Nenufar: The Coma Cluster Field

NenuFAR, the New Extension in Nancay Upgrading LOFAR, is currently in its early science phase. It is in this context that the Cosmic Filaments and Magnetism Pilot Survey is observing sources with the array as it is still under construction—with 57 (56 core, 1 distant) out of a total planned 102 (96 core, 6 distant) mini-arrays online at the time of observation—to get a first look at the low-frequency sky with NenuFAR. One of its targets is the Coma galaxy cluster: a well-known object, host of the prototype radio halo. It also hosts other features of scientific import, including a radio relic, along with a bridge of emission connecting it with the halo. It is thus a well-studied object.In this paper, we show the first confirmed NenuFAR detection of the radio halo and radio relic of the Coma cluster at 34.4 MHz, with associated intrinsic flux density estimates: we find an integrated flux value of 106.3 ± 3.5 Jy for the radio halo, and 102.0 ± 7.4 Jy for the radio relic. These are upper bound values, as they do not include point-source subtraction. We also give an explanation of the technical difficulties encountered in reducing the data, along with steps taken to resolve them. This will be helpful for other scientific projects which will aim to make use of standalone NenuFAR imaging observations in the future.