Dynamics and Morphology of Cold Gas in Fast, Radiatively Cooling
  Outflows: Constraining AGN Energetics with Horseshoes
  Warm ionized and cold neutral outflows with velocities exceeding $100\,{\rm
km\,s}^{-1}$ are commonly observed in galaxies and clusters. Theoretical
studies however indicate that ram pressure from a hot wind, driven either by
the central active galactic nucleus (AGN) or a starburst, cannot accelerate
existing cold gas to such high speeds without destroying it. In this work we
explore a different scenario, where cold gas forms in a fast, radiatively
cooling outflow with temperature $T\lesssim 10^7\,{\rm K}$. Using 3D
hydrodynamic simulations, we demonstrate that cold gas continuously fragments
out of the cooling outflow, forming elongated filamentary structures extending
tens of kiloparsecs. For a range of physically relevant temperature and
velocity configurations, a ring of cold gas perpendicular to the direction of
motion forms in the outflow. This naturally explains the formation of
transverse cold gas filaments such as the blue loop and the horseshoe filament
in the Perseus cluster. Based on our results, we estimate that the AGN outburst
responsible for the formation of these two features drove bipolar outflows with
velocity $>2,000\,{\rm km\,s}^{-1}$ and total kinetic energy
$>8\times10^{57}\,{\rm erg}$ about $\sim10$ Myr ago. We also examine the
continuous cooling in the mixing layer between hot and cold gas, and find that
radiative cooling only accounts for $\sim10\%$ of the total mass cooling rate,
indicating that observations of soft X-ray and FUV emission may significantly
underestimate the growth of cold gas in the cooling flow of galaxy clusters.
