A New Limit to the Size of the Radio Nucleus of NGC 5128

We have measured the size of the compact radio nucleus of the giant radio galaxy Centaurus A (NGC 5128) as 0.5 ± 0.1 milliarcseconds. The corresponding linear dimensions of 0.01 pc ~ 10 light-days ~ 1016 cm make this the smallest known extragalactic radio source. If the radio lobes are powered by a massive central engine, such as a black hole, their large total energy content combined with our measurement of the size of the central radio source, suggest that the central mass density may have exceeded 5 × 1013 M☉ pc-3, a value far larger than has been determined for any other active galactic nucleus or quasar.