Broad-line region

The region of rapidly moving gas near an active galactic nucleus that produces broad emission lines when illuminated by its central source.

Active galactic nucleus: A galaxy’s central region powered by accretion onto a supermassive black hole, often emitting intense radiation. Its radiation illuminates the gas, while its gravity and outflows shape the region.
Supermassive black hole: A black hole with a mass millions to billions of times that of the Sun. It powers the central source and dominates gravity close to the region.
Single-epoch virial mass estimator: A black-hole mass estimate using a broad-line width and continuum luminosity from one observation. It turns the region’s line width and inferred radius into a practical mass estimate.
Broad-line region geometry: The three-dimensional spatial arrangement of gas that emits broad lines around an active galactic nucleus. Line profiles and reverberation constrain geometry but rarely determine it uniquely.
Doppler broadening: The widening of spectral lines caused by motion along the line of sight. Large gas velocities spread emission across broad wavelengths.
Accretion disk: A rotating disk of matter spiraling toward a central object. Its radiation supplies much of the ionizing light that reaches the gas.
Black hole mass: The mass of a black hole, inferred through its gravitational effects or emitted radiation. Broad-line measurements provide one of the main mass estimates for distant active galaxies.
Broad-line region kinematics: The motions of gas producing broad emission lines, including rotation, inflow, and outflow. Different velocity patterns can create similar observed line profiles.
Photoionization: The removal of electrons from atoms or ions by absorbed photons. Radiation from the central source ionizes the gas and drives its emission lines.
Emission line: A discrete spectral feature produced when atoms or ions emit photons at characteristic wavelengths. Broad emission lines are the observable signature used to identify this gas.

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