GALAXY CLUSTERS AND THEIR CENTRAL SUPERMASSIVE BLACK HOLES: CASE OF M87

摘要

Observations suggest that AGN activity regulates the thermal state of the gas by injecting energy into the intra-cluster medium in the cores of relaxed clusters, where radiative cooling time is often as short as few 108 years. Bubbles of relativistic plasma are inflated by a supermassive black hole and rise buoyantly through the gaseous atmosphere, leading to a number of spectacular phenomena like expanding shocks, X-ray dim and radio bright cavities, X-ray dim and radio dim "ghost" cavities (aged version of "normal" cavities), filaments in the wakes of the rising bubbles formed by the entrained low entropy gas, etc. Simple estimates of the energetics involved (based on the estimates of the energy content of bubbles/cavities and their life-time) suggest that amount of mechanical energy supplied by AGNs is sufficient to offset gas cooling losses in objects vastly different in size and luminosity. This hints on some form of self-regulation controlling the AGN power as the gas cools or gets heated. One can build a toy model where accretion rate (and therefore the amount of energy provided by the AGN) is sensitive to the gas properties, in particular to its entropy, thus closing the feedback loop. How the mechanical energy, provided by the AGN, is dissipated depends on the ICM microphysics (e.g. magnetic fields, viscosity, conduction etc). However it is easy to imagine the situation when close to 100% of mechanical energy is eventually dissipated in the cluster core, regardless of the particular physical process involved. Comparison of the gravitational potential profiles of the elliptical galaxies derived from X-ray and optical data suggests that the combined contribution of cosmic rays, magnetic fields and micro-turbulence to the gas pressure is of order 10-30%. This in turn suggests that the dissipation time scale of the energy deposited by the AGN is a similar 10-30% fraction of the gas cooling time. The same process of AGN-ICM interaction, operating in nearby clusters, could be important at z = 2 - 3 when present day massive ellipticals were forming. The importance of this process depends critically on the physics of accretion. An analogy with the Galactic stellar mass black holes suggests that a black hole can switch from the radiation dominated mode to the mechanically dominated mode when the accretion rate drops below the fraction 10-2 - 10?1 of the Eddington value. Given that the coupling constant of these two forms of energy output with the ICM can differ by a factor of 104 - 105 this change in the accretion mode may explain the switch of a SMBH (and its parent galaxy) from the QSO-type behavior and an intense star formation to the radiatively inefficient AGN and essentially passive evolution of the parent galaxy.