TOWARD A COMPREHENSIVE MODEL FOR FEEDBACK BY ACTIVE GALACTIC NUCLEI: NEW INSIGHTS FROM M87 OBSERVATIONS BY LOFAR, FERMI , AND H.E.S.S

Feedback by active galactic nuclei (AGNs) appears to be critical in balancing radiative cooling of the low-entropy gas at the centers of galaxy clusters and in mitigating the star formation of elliptical galaxies. Low-frequency radio observations by LOFAR revealed the absence of fossil cosmic-ray (CR) electrons in the radio halo surrounding M87. This puzzle can be resolved by accounting for the CR release from the radio lobes and the subsequent mixing of CRs with the dense ambient intracluster gas, which thermalizes the electrons on a timescale similar to the radio halo age of 40 Myr. Provided that CR heating balances cooling during the emerging "cooling flow," the collapse of the majority of the gas is halted around 1 keV-in accordance with X-ray data. We show that both the existence of a temperature floor and the similar radial scaling of the heating and cooling rates are generic predictions of the CR heating model.