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An active galactic nucleus is the compact luminous centre of a galaxy powered mainly by black-hole accretion onto a supermassive black hole. Its radiation can dominate the host at some wavelengths even though the emitting region is small compared with the galaxy. Observers identify activity using combinations of emission lines, a compact radio wave source, energetic X-rays and brightness changes. Different components respond on different timescales: reverberation mapping measures the delay between changes in the central radiation and the response of surrounding line-emitting gas, giving a light-travel estimate of its distance. Combining that distance with the Doppler broadening of the emission lines constrains the central mass if the emitting gas is gravitationally bound. Activity can be episodic; the existence of a central black hole alone does not imply a luminous nucleus.^[ref]

John Kormendy investigates the structure of galaxies and their central supermassive black holes. His 2013 review with Luis C. Ho examines black hole mass measurements inferred from spatially resolved motion and their correlations with different components of the host galaxy.

An important distinction is between a galactic bulge, a galactic disk and a pseudobulge: the authors find that the same simple coevolution story does not describe all three. A correlation such as the M-sigma relation is therefore evidence to interpret in its structural context, rather than a universal description of every galaxy.^[ref]

Luis C. Ho investigates supermassive black holes and their connections to host galaxies. His review with John Kormendy organized dynamically measured black hole masses by host structure, including classical bulges, galactic disks and pseudobulges.

The distinction changes the physical interpretation of a scaling relation. Strong correlations with some bulge properties do not establish equally close coupling to every component of a galaxy. The review argued that weak relations for galactic disks and pseudobulges require a richer account of black hole growth than a single universal coevolution rule.^[ref]

Supermassive black holes are compact galactic nuclei with masses typically of millions to billions of solar masses. Their gravitational influence is measured from the motion of surrounding matter; accretion luminosity is evidence of feeding, not a necessary property of every black hole. Keplerian stellar orbits near the Galactic Centre and molecular-maser rotation curves in other nuclei provide particularly direct compact-mass constraints. A rough black-hole sphere of influence is $r_{\mathrm{infl}}\sim G{M}_{\bullet }/{\sigma }_{\star }^2$; resolving it helps dynamical mass estimates.

Accretion converts binding energy into radiation,The latter is the Eddington luminosity for isotropic radiation opposed by electron-scattering opacity, not an absolute luminosity ceiling for every geometry. The retained black-hole mass grows at $(1-\eta ){\dot{M}}_{\mathrm{in}}$. Sustained feeding can produce an active galactic nucleus, whereas gas-poor nuclei may be faint. Loss of gas angular momentum is a central feeding problem.

Black-hole mass correlates with classical-bulge stellar velocity dispersion through the M-sigma relation. Accretion-powered radiation, winds and jets can heat or remove gas, linking nuclear growth to galaxy evolution. Such correlations do not establish one universal feedback history, and disks and pseudobulges do not follow the same tight relations as classical bulges. Seed formation, accretion and mergers all contribute; seed masses and feeding histories remain model dependent.^[ref]