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