Which AGN Jets Quench Star Formation in Massive Galaxies?
  Without additional heating, radiative cooling of gas in the halos of massive
galaxies (Milky Way and above) produces cold gas or stars in excess of that
observed. Previous work suggested that AGN jets are likely required, but the
form of jet energy required to quench remains unclear. This is particularly
challenging for galaxy simulations, in which the resolution is orders of
magnitude coarser than necessary to form and evolve the jet. On such scales,
the uncertain parameters include: jet energy form (kinetic, thermal, and cosmic
ray (CR) energy), energy, momentum, and mass flux, magnetic field strength and
geometry, jet precession angle and period, opening-angle, and duty cycle. We
investigate all of these parameters in a $10^{14}\,{\rm M}_{\odot}$ halo using
high-resolution non-cosmological MHD simulations with the FIRE-2 (Feedback In
Realistic Environments) stellar feedback model, conduction, and viscosity. We
explore which scenarios match observational constraints and show that
CR-dominated jets can most efficiently quench the central galaxy through a
combination of CR pressure support and a modification of the thermal
instability. Jets with most energy in mildly relativistic ($\sim$ MeV or
$\sim10^{10}$ K) thermal plasma work, but require a factor $\sim 10$ larger
energy input. For a fixed energy flux, jets with higher specific energy (longer
cooling times) quench more effectively. For this halo size, kinetic jets are
less efficient in quenching unless they have wide opening or precession angles.
Magnetic fields play a minor role except when the magnetic flux reaches
$\gtrsim 10^{44}$ erg s$^{-1}$ in a kinetic jet model, which causes the jet
cocoon to significantly widen, and the quenching to become explosive. We
conclude that the criteria for a successful jet model are an optimal energy
flux and a sufficiently wide jet cocoon with long enough cooling time at the
cooling radius.
