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Summary We present VLA, MERLIN and VLBI observations at cm wavelengths of the radio galaxy 3C99, a low-frequency variable source with a steep radio spectrum. The radio observations show that the source consists of three principal components, which are roughly collinear and have steep radio spectra. Of these, the middle component is the brightest feature at the highest resolutions, has the lowest limits on the degree of polarization and is closest to the position of the optical galaxy. We associate this component with the nucleus of the galaxy and suggest that interstellar scattering of the radiation from it could be responsible for the observed low-frequency variability. The two outer components are very asymmetrically located relative to the nucleus, and also differ in peak brightness by a factor of about 70. We also present optical spectroscopic observations, made with the Isaac Newton Telescope, which show the presence of extended [O in] emission with a systematic velocity shift of the line across the source. We discuss possible relationships between the extended line emission and the radio structure.

The steep-spectrum low-frequency variable source 3C99: radio and optical observations