ON THE FORMATION OF SPIRAL AND RING-SHAPED FEATURES AROUND GALACTIC NUCLEI

This work is an improvement upon an earlier study about the formation of spiral features in galactic nuclei, starting again from an ejection mechanism [Pismis & Moreno, Afz., 20, 7 (1984)]. We now consider a se miballistic treatment to follow the motion of a mass element ejected f rom a nuclear region, and find the locus at a given time, and its asso ciated velocity field, on which the ejected elements are distributed. Given enough time, the interaction of the ejecta with the gas in the g alactic disk drives a spiral feature into a ring. After considering so me properties emerging from the model, we apply it to the galaxy NGC 4 736.