THE IONIZED-GAS IN NGC 5128 - EVIDENCE FOR A SHOCK-HEATED COMPONENT

New spectrophotometric and kinematic data for the ionized gas in the main body of the giant radio galaxy NGC 5128 are presented. An analysis of the emission-line spectra shows that two basic types of gas are present: (1) discrete and diffuse low-ionization Hn regions which are photo-ionized by the radiation of normal 0 and B stars, and (2) a turbulent, diffuse component which emits a spectrum more typical of shock-ionized nebulae. The low-ionization gas is more or less confined to the general vicinity of the dust lane, while the shock-ionized component is found at the nucleus and over a large region at the edge of the dust lane in the south-west half of the main elliptical body of the galaxy. Circumstantial evidence suggests that the shocked gas covers an even greater area of the elliptical component and may be connected with the extended X-ray emission that has recently been identified. Alternatively, the two components of ionized gas may have both naturally resulted from the infall of a gas-rich spiral galaxy. © Royal Astronomical Society.