Dual emission-line regions in the Seyfert galaxy NGC 5929

Abstract

Several theoretical and empirical studies suggest that multiple distinct emission regions exist in the nuclei of some active galaxies. Detailed study1,2 of nearby objects such as M51 shows that the most important radiation of nonstellar origin can originate hun-dreds of parsecs from the nucleus (as defined by the stellar potential well), while binary compact objects are expected in some cluster-collapse schemes3 or in interaction-fuelled models for active nuclei4,5. Here I show that the type 2 Seyfert nucleus of NGC5929 contains two emission regions situated symmetrically about the centre of the galaxy. Slit spectroscopy shows them to be spatially as well as kinematically distinct, each with a linewidth of ∼200 km s−1 and ionization levels normally associated with Seyfert nuclei. Their location in the galaxy's rotation curve suggests that they move with the galaxy's disk gas; and they probably result from energy transport from a central object of low photon luminosity but high energy output. This system furnishes a very clear example of the presence of phenomena related to the presence of a central engine, but located at a significant distance from the ultimate source of energy. The existence of such objects suggests that the usual radiatively powered, nearly symmetric models for active nuclei may be misleading in many objects.