Cosmic Ray-Dominated AGN Jets and the Formation of X-ray Cavities in Galaxy Clusters

Abstract: It is widely accepted that feedback from active galactic nuclei (AGN) plays a key role in the evolution of gas in groups and clusters of galaxies. Unequivocal evidence comes from quasi-spherical X-ray cavities observed near cluster centers having sizes ranging from a few to tens of kpc, some containing radio emission. Cavities apparently evolve from the interaction of AGN jets with the intracluster medium (ICM). However, in numerical simulations it has been difficult to create such fat cavities from narrow jets. Ultra-hot thermal jets dominated by kinetic energy typically penetrate deep into the ICM, forming radially elongated cavities at large radii unlike those observed. Here, we study very light jets dominated energetically by relativistic cosmic rays (CRs) with axisymmetric hydrodynamic simulations, investigating the jet evolution both when they are active and when they are later turned off. We find that, when the thermal gas density in a CR-dominated jet is sufficiently low, the jet has a correspondingly low inertia, and thus decelerates quickly in the ICM. Furthermore, CR pressure causes the jet to expand laterally, encounter and displace more decelerating ICM gas, naturally producing fat cavities near cluster centers similar to those observed. Our calculations of cavity formation imply that AGN jets responsible for creating fat X-ray cavities (radio bubbles) are very light, and dominated by CRs. This scenario is consistent with radio observations of Fanaroff-Riley I jets that appear to decelerate rapidly, produce strong synchrotron emission and expand typically at distances of a few kpc from the central AGN.