  Simple arguments suggest that magnetic fields should be aligned tangentially
to the surface of an AGN-blown bubble. If this is the case, charged particles
from the fully ionised intra-cluster medium (ICM) will be prevented,
ordinarily, from crossing the boundary by the Lorentz force. However, recent
observations indicate that thermal material may occupy up to 50% of the volume
of some bubbles. Given the effect of the Lorentz force, the thermal content
must then be attributed to one, or a combination, of the following processes:
i) the entrainment of thermal gas into the AGN outflow that inflated the
bubble; ii) rapid diffusion across the magnetic field lines at the ICM/bubble
interface; iii) magnetic reconnection events which transfer thermal material
across the ICM/bubble boundary. Unless the AGN outflow behaves as a magnetic
tower jet, entrainment may be significant and could explain the observed
thermal content of bubbles. Alternatively, the cross-field diffusion
coefficient required for the ICM to fill a typical bubble is roughly 10^16 cm^2
s^-1, which is anomalously high compared to predictions from turbulent
diffusion models. Finally, the mass transfer rate due to magnetic reconnection
is uncertain, but significant for plausible reconnection rates. We conclude
that entrainment into the outflow and mass transfer due to magnetic
reconnection events are probably the most significant sources of thermal
content in AGN-blown bubbles.
