A LOFAR view of three HETDEX galaxy clusters: radio halos and their connection with X-ray emitting thermal gas

Radio halos (RHs) are found in a growing number of galaxy clusters. According to the widely accepted scenario, the emitting particles are re-accelerated in the cluster magnetic fields during cluster merger events. One of the most important evidence supporting this scenario is that RHs are typically hosted in dynamically active systems. The study of the connection between the cluster dynamics and the properties of RHs is crucial to constrain the origin of relativistic particles in galaxy clusters. This is the approach followed in this Thesis. We use low-frequency data from the new generation radio telescope LOFAR. In particular, we analyzed radio and X-ray data of three galaxy clusters: Abell 1550, RXC J1115.2+5320 and Abell 1622. These are part of the HETDEX field in the LOFAR LoTSS survey at 144 MHz. The last two targets were never studied in the radio band. Abell 1550 was already studied at 1.4 GHz and was known to host a RH. By combining VLA and LOFAR data we measure a very steep spectrum of this RH supporting turbulent reacceleration for the radio emitting electrons. We discovered a RH in RXC J1115.2+5320, while no diffuse emission is detected in Abell 1622 by the LOFAR observations. For Abell 1622, that is the smaller cluster in our sample, we derived an upper limit and demonstrate that it is underluminous in radio. We suggest that this is due to the smaller energy budget available to particles reacceleration in less massive systems like Abell 1622. We studied the connection between thermal (X-rays) and non thermal (radio) properties in our clusters. Specifically we: -explore the dynamics of our targets measuring dynamical parameters; -investigate on the behaviour of these clusters in the correlations between radio luminosity and mass (or X-ray luminosity); -derive point-to-point radio brightness - X-ray brightness correlations for the RHs in Abell 1550 and RXC J1115.2+5320. All these findings are discussed in the context of current theoretical models.