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Radio-mode AGN feedback plays a key role in the evolution of galaxy groupsand clusters. Its physical origin lies in the kpc-scale interaction of AGN jetswith the intracluster medium. Large-scale jet simulations often initiate lightinternally-supersonic jets with density contrast $0.01<\eta<1$. Here we arguefor the first time for the importance of very-light ($\eta<0.01$)internally-subsonic jets. We investigated the shapes of young X-ray cavitiesproduced in a suite of hydrodynamic simulations, and found that bottom-widecavities are always produced by internally-subsonic jets, whileinternally-supersonic jets inflate cylindrical, center-wide, or top-widecavities. We found examples of real cavities with shapes analogous to thoseinflated in our simulations by internally-subsonic and internally-supersonicjets, suggesting a dichotomy of AGN jets according to their internal Machnumbers. We further studied the long-term cavity evolution, and found that oldcavities resulted from light jets spread along the jet direction, while thoseproduced by very light jets are significantly elongated along the perpendiculardirection. The northwestern ghost cavity in Perseus is pancake-shaped,providing tentative evidence for the existence of very light jets. Oursimulations show that very-light internally-subsonic jets decelerate faster andrise much slower in the ICM than light internally-supersonic jets, possiblydepositing a larger fraction of jet energy to cluster cores and alleviating theproblem of low coupling efficiencies found previously. The internal Mach numberpoints to the jet's energy content, and internally-subsonic jets areenergetically dominated by non-kinetic energy, such as thermal energy, cosmicrays, or magnetic fields.

On the Importance of Very-light Internally-subsonic AGN Jets in Radio-mode AGN Feedback