Radioloud

Radio-loud refers to a class of active galactic nuclei and galaxies that emit strongly in the radio part of the spectrum due to relativistic jets powered by a supermassive black hole. In these systems, jet- and lobe-related synchrotron radiation dominates the radio band, while other wavelengths may be comparatively weaker. Radio-loud objects are contrasted with radio-quiet AGN, which show comparatively weak jet-related radio output.

Radio-loudness is commonly quantified by the ratio of radio to optical flux densities, such as R = F_radio(5 GHz)/F_optical(B). A threshold often used is log R > 1 (R > 10), but definitions vary with wavelength, redshift, and survey. Some use radio luminosity thresholds (e.g., L_radio > 10^25 W Hz^-1). Consequently the boundary is not universal and depends on measurement method and sample selection.

The radio emission originates from relativistic jets and extended lobes, produced by synchrotron radiation. Jet orientation affects observed properties: when a jet is closely aligned with our line of sight, objects may appear as blazars with rapid variability and polarization; misaligned jets are seen as radio galaxies or radio-loud quasars.

Subclasses include radio-loud quasars, radio galaxies, and blazars (including BL Lac objects and flat-spectrum radio quasars). Many radio-loud AGN reside in giant elliptical galaxies and can influence their environments through jet-driven feedback, heating interstellar and intracluster gas.

Radio surveys (such as FIRST, NVSS, and LOFAR) and multiwavelength campaigns continue to refine the demographics, physics, and evolution of radio-loud sources, contributing to understanding jet formation, black hole growth, and galaxy evolution.