The Radio Properties of Brightest Cluster Galaxies

Abstract

Energetic feedback from the Active Galactic Nucleus (AGN) of the Brightest Cluster Galaxy (BCG) is required to prevent catastrophic cooling of the intra-cluster medium (ICM) in galaxy clusters. Evidence for this is seen through the inflation of cavities in the ICM by AGN-launched, radio-emitting jets, and understanding this process is an active area of research. Radio observations play an integral role in this, as they trace the active stages of the feedback cycle. Understanding the radio properties of BCGs is therefore paramount for understanding both galaxy clusters and AGN feedback processes globally.

Within this thesis, the BCGs in a large (700) sample of X-ray selected clusters are studied. We observe these BCGs with a wide variety of facilities, building a census of their radio properties across a range of frequencies, timescales and angular resolutions. Radio spectral energy distributions (SEDs) are built for over 200 BCGs, and then decomposed into two components; a core, attributable to ongoing nuclear activity, and a non-core, attributable to historical accretion. Both components are not only more common, but also significantly more powerful in cool-core (CC) clusters than non-cool core (NCC) clusters. However, it is the presence of an core that shows BCGs in CC clusters are constantly `on' - explaining how they regulate their environments over gigayear timescales.

We observe 35 currently active BCGs at high (15~--~353~GHz) radio frequencies, and monitor their variability. Self-absorbed, active components are found to be common at high frequency. Little variability is seen on year timescales, although longer term variation of 10 annually over few-decade timescales is observed. Evidence is presented for a hitherto unseen component in BCG spectra that may be attributable to a naked Advection Dominated Accretion Flow (ADAF). The milli-arcsecond scale radio properties of 59 sources are studied, with a large range of morphologies recovered although no evidence is found for dual AGN being common in BCGs.

Finally, we present a study that has more than doubled the number of HI absorption systems known in BCGs. We show that both the detection rate and column densities observed are strongly affected by the multi-scale properties of the radio continuum. All our clear detections are redshifted or at the systemic velocity. Most HI appears to be located in a clumpy torus that is replenished by residual material cooling from the ICM, linking the environment to the central engine and completing the feedback cycle.