Migration of Star Clusters and Nuclear Rings
  Star clusters that form in nuclear rings appear to be at slightly larger radii than the gas. We argue that the star clusters move out from the gas in which they are formed because of satellite-disk tidal interactions. In calculating the dynamics of this star cluster and gas ring system, we include the effects of dynamical friction of the background stars in the host galaxy on the star cluster, and inflowing gas along the bar onto the nuclear ring at the two contact points. We show that the final separation is of the order of the Hill radius of the nuclear ring, which is typically 20-30% of its radius. Massive star clusters can reach half of this separation very quickly and produce a factor of a few enhancement in the gas surface density. If this leads to star formation in addition to the (ongoing) formation of star clusters near the contact points, a possible (initial) azimuthal age gradient may become diluted or even disappear. Finally, if the star cluster are massive and/or numerous enough, we expect the nuclear ring to migrate inward, away from the (possibly) associated (inner) Lindblad resonance. We discuss how these predictions may be tested observationally. 
Glenn van de Ven (Institute for Advanced Study), Philip Chang   (University of California, Berkeley)