Shocked gas and star formation in the Centaurus A radio galaxy

Observational evidence for shock-induced star formation is found in the northeast radio lobe of the nearby radio galaxy Centaurus A (NGC 5128). A gas cloud, recently detected in H I, is impacted by the adjacent radio jet to the extent that cloud collapse is triggered and loose chains of blue supergiant stars are formed. Diffuse clouds and filaments of ionized gas have been observed near the interface of the H I cloud and the radio jet. These show velocities that cover a range of more than 550 km s(-1). Line intensities in their spectra are characteristic of a shock-related origin with strong [N II] and [S II] relative to H alpha. The [O III]/H alpha line ratio indicates a large range in excitation that is not correlated with velocity. Distinct from this component is a group of four apparently normal H II regions that are excited by embedded young stars and whose velocities are very close to that of the H I cloud. Star formation will continue for as long as the gas cloud remains close to the radio jet. The loose chains of blue stars in the area are resolved only because NGC 5128 is so close. The reported faint blue extensions and plumes in more distant analogs probably have similar origins.