Modelling cosmic rays at AGN jet-driven shock fronts

ABSTRACT Feedback from active galactic nuclei (AGNs) is a key physical mechanism proposed to regulate galaxy formation and suppress star formation, primarily in massive galaxies. Cosmic rays (CRs) associated with AGN jets can efficiently suppress cooling flows and quench star formation, but the locus of CR production and their coupling to gas are crucial to self-regulation. We conduct high-resolution, non-cosmological magnetohydrodynamic (MHD) simulations of a massive $10^{14} {\rm M_\odot }$ halo using the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We explore AGN jet feedback with CRs by varying the CR energy fraction in jets, the CR coupling sites (in the vicinity of the black hole versus at the shock fronts of large-scale jet cocoons), and jet precession parameters. Our findings indicate that injecting CRs near the black hole efficiently inhibits accretion by lowering the local gas density before the jet propagates to large radii. This produces episodic accretion and leaves the jet with insufficient energy flux to reach large radii and impact cooling flows. By contrast, injecting CRs at the shock front of the jet cocoon sustains a higher jet energy flux for longer and disperses CRs to larger radii. This configuration more effectively suppresses the cooling flow. The period and angle of jet precession influence shock-front positions. We identify an optimal range of precession periods of order tens of Myr that places shocks in the inner circumgalactic medium (CGM), where cooling flows are most severe. We report that this configuration most effectively suppresses cooling flows and quenches star formation.