Observations of several Fanaroff–Riley ( FR ) type I sources reveal outflowing bipolar bubbles of hot gas surrounded by a weak forward shock .
We consider the possibility that these bubbles were driven by choked relativistic jets which failed to penetrate the ambient intracluster medium ( ICM ) .
Using new results on choked jets linking the geometry of the forward shock to the jet properties , we infer robust limits on the radius R _ { ch } at which the jet was quenched in 5 well-studied FRI sources , finding typically R _ { ch } \sim 10 kpc .
We further show that , in order to reach this radius in less than the current age of the system , the jet must have been tightly collimated , with the jet head subtending an angle of \theta _ { h } \la 2 \degree .
The ambient pressure is not high enough to explain this collimation , suggesting that the jet was collimated by interaction with its own cocoon .
Although the choking radius is well-constrained , we find a degeneracy between the initial jet opening angle before collimation , \theta _ { 0 } , and the duration of jet activity , t _ { b } , with ( t _ { b } / 1 ~ { } Myr ) ( \theta _ { 0 } / 5 \degree ) ^ { -2 } \sim 0.1 .
We speculate that the working time and/or opening angle of the jet may be important factors contributing to the FR type I/type II morphology in galaxy clusters , with short-lived or wide jets being choked to form bipolar bubbles filled with diffuse radio emission , and longer-lived or narrow jets successfully escaping the cluster core to produce cocoons with radio hotspots .