Pattern speed evolution and bar reformation
  Bars in spiral galaxies can weaken through gas inflow towards the center, and angular momentum transfer. Several bar episodes can follow one another in the life of the galaxy, if sufficient gas is accreted from the intergalactic medium to revive young disks.   Pattern speeds of the successive bars are different, due to mass concentration, or increased velocity dispersion of the remaining stellar component. In the same time, the spiral galaxy evolves in morphological type. Numerical simulations are presented, trying to correlate type and bar pattern speeds. 
F. Combes (LERMA, Observatoire de Paris)