A radio galaxy (RG) is a galaxy emitting a lot of radio electromagnetic radiation (i.e., radio-loud; The phrase radio-quiet is sometimes used to distinguish radio-loud objects from others in discussion). Instances have been found apparently emitting as much as 1039 watts of power in the radio spectrum. Distant radio galaxies are useful as they are relatively easily observed. Some emit most of their radio emission from regions away from the center of the galaxy (often even beyond the visible portion of the galaxy), the regions being termed lobes. The spectral energy distribution and polarization often suggests synchrotron radiation as the source. An SFRG (star forming radio galaxy) has rest-frame ultraviolet spectral features consistent with a starburst galaxy. The term GRG (giant radio galaxy) naturally is used to indicate a large one, 2 Mpc diameter being a commonly-used threshold. The terms PRG (powerful radio galaxy) and WRG (weak radio galaxy) are also used.
A radio galaxy is presumed to be an active galaxy. Quasars and blazars with a similar radio spectrum are called radio-loud quasars and radio-loud blazars and are assumed to be more distant radio-loud radio galaxies. The mechanism of radio emission is thought to involve the active galactic nucleus (AGN), and the lobes from interaction between jets and the surrounding intergalactic medium (IGM), the jets being directed along the SMBH axis of rotation. An X-shaped radio galaxy (aka X-shaped galaxy or winged radio galaxy) shows four lobes along two lines, forming an X. The mechanism for this is a subject of research, theories being two active SMBHs out of alignment, or a single SMBH whose rotation axis has been changed, or a deflection of the jets that began at some time, with the jet-output along both axes still visible.