Feedback under the microscope – II. Heating, gas uplift and mixing in the nearest cluster core

Using a combination of deep (574 ks) Chandra data, XMM-Newton high-resolution spectra and optical Hα+[N II] images, we study the nature and spatial distribution of the multi-phase plasma in M87. Our results provide direct observational evidence of 'radio-mode' active galactic nuclei (AGN) feedback in action, stripping the central galaxy of its lowest entropy gas and therefore preventing star formation. This low entropy gas was entrained with and uplifted by the buoyantly rising relativistic plasma, forming long 'arms'. A number of arguments suggest that these arms are oriented within 15°-30° of our line-of-sight. The mass of the uplifted gas in the arms is comparable to the gas mass in the approximately spherically symmetric 3.8 kpc core, demonstrating that the AGN has a profound effect on its immediate surroundings. The coolest X-ray emitting gas in M87 has a temperature of ~0.5 keV and is spatially coincident with Hα+[N II] nebulae, forming a multi-phase medium where the cooler gas phases are arranged in magnetized filaments. We place strong upper limits of 0.06 M ⊙ yr -1 (at 95 per cent confidence) on the amount of plasma cooling radiatively from 0.5 to 0.25 keV and show that a uniform, volume-averaged heating mechanism could not be preventing the cool gas from further cooling. All of the bright Hα filaments in M87 appear in the downstream region of the <3 Myr old shock front, at smaller radii than ~0.6 arcmin. We suggest that shocks induce shearing around the filaments, thereby promoting mixing of the cold gas with the ambient hot intra-cluster medium (ICM) via instabilities. By bringing hot thermal particles into contact with the cool, line-emitting gas, mixing can supply the power and ionizing particles needed to explain the observed optical spectra. Furthermore, mixing of the coolest X-ray emitting plasma with the cold optical line-emitting filamentary gas promotes efficient conduction between the two phases, allowing non-radiative cooling which could explain the lack of X-ray gas with temperatures under 0.5 keV.