Stopping Cooling Flows with Cosmic Ray Feedback

Abstract

Multi-Gyr two-dimensional calculations describe the gasdynamical evolution of hot gas in the Virgo cluster resulting from intermittent cavities formed with cosmic rays. Without cosmic rays, the gas evolves into a cooling flow, depositing about 85 solar masses per year of cold gas in the cluster core -- such uninhibited cooling conflicts with X-ray spectra and many other observations. When cosmic rays are produced or deposited 10 kpc from the cluster center in bursts of about 10^{59} ergs lasting 20 Myrs and spaced at intervals of 200 Myrs, the central cooling rate is greatly reduced to 0.1 - 1 solar masses per year, consistent with observations. After cosmic rays diffuse through the cavity walls, the ambient gas density is reduced and is buoyantly transported 30-70 kpc out into the cluster. Cosmic rays do not directly heat the gas and the modest shock heating around young cavities is offset by global cooling as the cluster gas expands. After several Gyrs the hot gas density and temperature profiles remain similar to those observed, provided the time-averaged cosmic ray luminosity is about 10^{43} erg/s, approximately equal to the bolometric X-ray luminosity within only 56 kpc. If an appreciable fraction of the relativistic cosmic rays are protons, gamma rays produced by pion decay following inelastic p-p collisions may be detected with the Fermi Gamma Ray Telescope.