The work presented in this thesis is divided into two major topics: (1) observations of diffuse radio emission in clusters, and (2) the study of the gravitational lensing effect in massive cluster cores. In the first part of the dissertation, we extend the number of radio cluster observations fo- cused on detecting diffuse cluster emission with a SZ mass-selected sample from ACT. In chapter 2 we present the observational properties of the three main types of diffuse radio emission found in clusters, and discuss the existing theoretical models for the formation mechanism responsible for the emission. In chapter 3 we present our cluster sample and discuss our new radio observa- tions and data reduction process. Finally, we present the results of our radio observations in the context of the detection of diffuse radio emission.
In chapter 4, we discuss the radio properties of the known merging cluster in our sample, ACT-CL J0256.5+0006, which we found to host a faint giant radio halo. Furthermore, we present a multi-wavelength analysis of the cluster using X-ray and spectroscopic redshift data. Finally, we model the cluster merger as a two-body gravitational interaction and use the multi-wavelength data to constrain the merger geometry and merger time-scale. We compare our results with simulations to better understand the halo properties we observe.
The second part of this thesis presents high precision mass reconstructions through gravita-
tional lensing studies of two of the Hubble Frontier Field clusters. In chapter 5 we introduce gravitational lensing theory, and apply it to massive galaxy clusters, introducing both the strong and the weak regimes. Furthermore, we discuss lens modelling techniques and focus on the parametric approach implemented in the publicly availableLENSTOOLsoftware. In chapter 6 we detail the strong lensing analyses of MACSJ0416.1-2403 and MACSJ1149.6+2223 based on the new data from the Hubble Frontier Fields project, and present the resulting high-precision lensing mass maps. For MACSJ0416.1-2403 the strong lensing analysis is combined with weak lensing measurements to produce a lensing mass reconstruction out to larger radii. This combined lens- ing analysis is used in conjunction with multi-wavelength data to constrain the merger history and dynamics of this complex cluster. The strong lensing analysis of MACSJ1149.6+2223 allows us to estimate time delays and future appearance predictions for the lensed supernova observed in one of the multiply lensed images. Finally, in chapter 7 we summarise our results and describe future extensions to the work presented here.
We adopt aΛCDM flat cosmology withH0 = 70km s−1Mpc−1throughout this thesis, how- ever the assumed cosmological parameters differ slightly between the two parts of the thesis. For the radio portion we adopt values ofΩm= 0.27 andΩΛ= 0.73. We assumeSν ∝ν−αthroughout the radio analysis, whereSν is the flux density at frequencyν andαis the spectral index. In the second part of the thesis, we adopt values ofΩm= 0.3 andΩΛ= 0.7. All source magnitudes from the Hubble imaging are quoted in the AB system.
DIFFUSE RADIO EMISSION IN GALAXY CLUSTERS
In the hierarchical model of structure formation, the formation of galaxy clusters through mergers is one of the most energetic processes in the Universe, dissipating large amounts of energy (∼
1064 erg) into the intracluster medium (ICM). The non-thermal processes in the ICM can be probed via observations of diffuse radio emission in the cluster environment of which there are several classes. In this chapter we discuss the observational properties of each type and the various models to explain the physical processes which drive them.
2.1 Synchrotron radiation in terms of diffuse radio emission
When electrons are radially accelerated to relativistic speeds (when the acceleration is perpen- dicular to the velocity) in a magnetic field, the particles emit synchrotron radiation (see Figure 2.1). The emitted power is given by the relativistic Larmor formula and thus depends on the Lorentz factor (which defines the electron energy) and the magnitude of the magnetic field. Large magnetic fields require less electron energy in order to produce emission at a given frequency
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Figure 2.1: Schematic of synchrotron radiation emitted from an electron/proton spiraling in a magnetic field. In the case of synchrotron emission, the particle acceleration is perpendicular to its velocity. Source: adapted fromhttp://abyss.uoregon.edu
compared to magnetic fields of lower strengths. Thus for a given electron energy, the stronger the magnetic field, the more powerful the synchrotron radiation will be.
Thorough treatments of radiation processes and synchrotron theory can be found in several textbooks (e.g. Rybicki and Lightman, 1986; Longair, 2011). Here we summarise the features of synchrotron emission in an astrophysical context. Consider a homogeneous and isotropic population of relativistic electrons (CRe) with a power-law energy distribution of indexδgiven by
N(E)dE ∝E−δdE. (2.1)
If this plasma is also optically thin, which is the case for diffuse radio emission, the radiated synchrotron emission will exhibit the following properties:
• At a particular frequency, the emissivity, which is related to the number density of CRe and the strength of the magnetic field, follows a power law with a spectral index,α, related to the CRe energy distribution index:α= (δ−1)/2. Typical radio sources exhibit spectral indices of∼0.7 - 0.8.
• Over time, CRe experience energy losses which leads to a change in the global energy
distribution of the particles, and therefore a change in the spectral profile. Strong energy losses produce a cut-off of the spectrum at frequencies higher than a critical frequency ν∗, which is linked to the particle lifetime. Therefore older radio sources exhibit curved spectra and generally have steeper spectral indices than the typical sources. The spectrum at lower frequencies indicates the original energy distribution of the CRe.
• If the CRe are spiraling in a uniform magnetic field, the emitted synchrotron radiation is linearly polarized with the electric vector perpendicular to the plane-of-the-sky projection of the magnetic field. The degree to which the emission is intrinsically polarized depends on the energy distribution. For typical spectral indices this is 75 - 80%. The degree of polarization is reduced when the magnetic fields involved have complex or tangled struc- tures.
• As mentioned above, the total energy of a synchrotron source is defined by the energies of the relativistic particles as well as the magnetic field, taking into account the magnetic field filling factor (the fraction of the source volume occupied by the magnetic field). Obser- vations of synchrotron radio sources enable a determination of the minimum total energy under the equipartition condition. This is when the energy contributions are more or less equally split between the accelerated particles and the magnetic field. The magnetic field in this case is generally called the equipartition field,Beq.
Over the past two decades, observations of cluster-scale diffuse radio emission of synchrotron origin have been made. These sources can be separated into different classes but all of them exhibit low surface brightnesses and steep spectra, have no obvious link to the cluster galaxies, and are thus associated with the ICM (see the reviews by Ferrari et al., 2008; Feretti et al., 2012, and references therein). The prevailing theory for the origin and distribution of cluster magnetic fields is that they originate from cosmological fields or are injected by active galactic nuclei (AGN), and are then amplified through the hierarchical build up of structure (see Ryu et al., 2008; Roettiger et al., 1999; Dolag et al., 2002, 2005a; Subramanian et al., 2006; Xu et al., 2011, for more details).
As the diffuse radio emission is both extended and of low surface brightness, it can be difficult to detect. Successful observations require both excellent surface brightness sensitivity, typical of single dish telescopes, as well as good angular resolution to distinguish compact sources embed- ded in or projected along the line of sight of the emission. Interferometers are better suited to diffuse emission studies, as single dish instruments have large beams and often suffer from con- fusion. However, many interferometers do not have sufficient sensitivity to structures on large scales as they do not sample many short spacings. Another challenge to fully understanding diffuse synchrotron sources is that information about the age and energy of the specific popula- tion of CRe is encoded in the shape of the spectrum, with sensitive multi-frequency observations required to reproduce this shape for a single source. This is rarely available for large numbers of observed sources. Finally, since the diffuse emission in question exhibit steep spectra, they are best identified at low frequencies. Radio campaigns with instruments such as the Westerbork Synthesis Radio Telescope (WSRT), the Very Large Array (VLA), and the Giant Metrewave Ra- dio Telescope (GMRT), with their low radio frequencies (<1.4 GHz), good angular resolution, and low surface brightness sensitivities, have increased the knowledge of diffuse radio structures in clusters over a relatively short period of time (Venturi et al., 2007, 2008; Giacintucci, 2011;
van Weeren et al., 2011d,b).