Bars within Bars in Galaxies
  Inner regions of barred disk galaxies often include asymmetrical, small-scale central features, some of which are best described as secondary bars. Because orbital timescales in the galaxy center are short, secondary bars are likely to be dynamically decoupled from the main kiloparsec-scale bars. We found that non-chaotic multiply-periodic particle orbits can exist in potentials with two dynamically decoupled bars.Stars trapped around these orbits could form the building blocks for a long-lived, doubly-barred galaxy. A self-consistent secondary bar appears to induce formation of inner gaseous rings rather than shocks in gas flow. 
Witold Maciejewski(1,2) and Linda S. Sparke (1) ((1) University of   Wisconsin, (2) MPIA Heidelberg)