Energy Release and Transport Processes in the Centres of Galaxies

Observations over the past decade have verified, beyond reasonable doubt, that most galactic nuclei contain massive black holes. Hole masses are being measured and firm evidence for spin is being sought. Attention is now returning to the study of how gas flows around black holes and how energy is released both from the accreting gas and from the hole itself in the form of radiation, relativistic jets and non-relativistic, hydromagnetic winds. Some of the different possibilities currently under investigation are briefly reviewed and some recent clues from radio scintillation, polarization and X-ray observations are discussed. It is argued that observations of persistent circular polarisation in Sgr A$^\ast$ support the presence of an ordered disk magnetic field. It is also conjectured that adiabatic, sub- and super-critical accretion flows are demand-limited, not supply-driven and are associated with large mass outflow as appears to be the case in Sgr A$^\ast$. This principle may have to be modified when a massive black hole forms in a protogalaxy and this modification may account for the proposed hole mass-bulge velocity dispersion relation. The final stages of this process may release sufficient wind energy from the nucleus to prevent disk formation.