Speaker
Description
Galaxy clusters can host significant quantities of diffuse radio emission. The origin of this radio emission is still poorly understood. However, low frequency radio observations can help to shine light on the acceleration mechanisms that produce such diffuse emission. In particular, galaxy cluster mergers can re-accelerate seed electrons to extreme energies. Radio observations below 30 MHz are predicted to reveal the origin and nature of the population of these seed electrons. Radio observations below 30 MHz are very rare, however, due to the severe perturbing effects of the ionosphere. Recent calibration innovations with the LOw Frequency ARray (LOFAR) have enabled us to study galaxy clusters with high sensitivity below 30 MHz. This represents more than an order of magnitude improvement over previous decameter studies, both in terms of resolution (45 arcseconds) and sensitivity (12 mJy RMS noise). We have identified four fossil plasma sources corresponding to galaxy clusters in the region surveyed. These rare sources are believed to contain old, possibly re-energised, radio plasma originating from previous outbursts of active galactic nuclei. Notably, two of these sources display the steepest radio spectral index among all the sources detected (ɑ=-1.8 and -1.4). This has profound consequences for the understanding of the origin of non-thermal plasma in galaxy clusters.
