On the interaction of FR-II radio sources with the intracluster medium

The effect of the expansion of powerful FR-II radio sources into a cluster environment is discussed. The analysis considers both the thermal and temporal evolution of the ICM which has passed through the bow shock of the radio source and the effect of this swept-up gas on the dynamics of the radio source itself. The final state of the swept-up ICM is critically dependent on the thermal conductivity of the gas. If the gas behind the bow shock expands adiabatically, and the source is expanding into a steeply falling atmosphere, then a narrow dense layer will form as the radio source lifts gas out of the cluster potential. This layer has a cooling time very much less than that of the gas just ahead of the radio source. This effect does not occur if the thermal conductivity of the gas is high, or if the cluster atmosphere is shallow. The swept-up gas also affects the dynamics of the radio source especially as it slows towards sub-sonic expansion. The preferential accumulation of the swept-up gas to the sides of the cocoon leads to the aspect ratio of the source increasing. Eventually the contact surface must become Rayleigh-Taylor unstable leading both to inflow of the swept-up ICM into the cavity created by the cocoon, but also substantial mixing of the cooler denser swept-up gas with the ambient ICM thereby creating a multi-phase ICM. The radio source is likely to have a marked effect on the cluster on timescales long compared to the age of the source.