VLBI Observations of a Complete Sample of Radio Galaxies. II. The Parsec-Scale Structure of NGC 315

We present multifrequency radio observations of the low-luminosity (Fanaroff-Riley type 1) radio galaxy NGC 315 at parsec resolution. The observations were obtained at 1.6,5, and 8.4 GHz with the European and US VLBI networks. In addition VLA observations were used to obtain arcsec resolution maps at the same frequencies. The radio morphology of NGC 315 is asymmetric at milliarcsec resolution. It shows nuclear emission with a core size of 0.05 pc, and a one-sided jet. We place a limit on the milliarcsec jet to counterjet brightness ratio of B_jet_/B_cjet_ >= 50 at 1.6 GHz. At arcsec resolution we see a counterjet which is shorter and much more diffuse than the main jet. We derive a spectral index profile at parsec resolution, and estimate the physical conditions in the parsec-scale jet. If we assume that the parsec-scale asymmetry is caused by Doppler boosting of an intrinsically identical jet-counterjet pair, the derived brightness ratio gives β cos θ >~ 0.54, which gives an angle a between the source axis and the line of sight <~ 58^deg^. However, Doppler boosting cannot account for the asymmetry observed on the arcsec scale, given the much lower velocity flow estimated for the arcsec jet. This, together with the lack of any visible motion of the features in the maps at different epochs, suggests that an intrinsic asymmetry in the nuclear engine of NGC 315 is likely to be responsible for the observed radio asymmetry.