Dissipation of AGN jets in a clumpy interstellar medium

Abstract: Accreting supermassive black holes (SMBHs) frequently power jets that interact with the interstellar/circumgalactic medium (ISM/CGM), regulating star-formation in the galaxy. Highly supersonic jets launched by active galactic nuclei (AGN) power a cocoon that confines them and shocks the ambient medium. We build upon the models of narrow conical jets interacting with a smooth ambient medium, to include the effect of dense clouds that are an essential ingredient of a multiphase ISM. The key physical ingredient of this model is that the clouds along the supersonic jet-beam strongly decelerate the jet-head, but the subsonic cocoon easily moves around the clouds without much resistance. We propose scalings for important physical quantities -- cocoon pressure, head & cocoon speed, and jet radius. We obtain, for the first time, the analytic condition on clumpiness of the ambient medium for the jet to dissipate within the cocoon and verify it with numerical simulations of conical jets interacting with a uniform ISM with embedded spherical clouds. A jet is defined to be dissipated when the cocoon speed exceeds the speed of the jet-head. We compare our models to more sophisticated numerical simulations, direct observations of jet-ISM interaction (e.g., quasar J1316+1753), and discuss implications for the Fermi/eROSITA bubbles. Our work also motivates effective subgrid models for AGN jet feedback in a clumpy ISM unresolved by the present generation of cosmological galaxy formation simulations.