THE LONG-TERM EVOLUTION OF BARRED GALAXIES

We investigate the long-term dynamical evolution of barred galaxies using flat, fully self-gravitating, bar-unstable, disc-halo models. Our models develop rapidly rotating bars which subsequently slow through roughly equal transfers of angular momentum to the outer disc and to the halo. Once our bars are well formed, their pattern speeds decline approximately linearly with time, and we conclude that even in the absence of visible spiral patterns, a low-amplitude trailing response can continue to slow bars significantly for approximately 10 Gyr. Our experiments suggest that a typical bar will slow by a factor of approximately 2 in a Hubble time. When our bars first form, they end at or slightly inside corotation (1.0 less-than-or-similar-to r(cr)/r(bar) less-than-or-similar-to 1.1), but this distance grows with time and, by the end of our simulations, r(cr)/r(bar) greater-than-or-similar-to 1.3. Such a ratio agrees well with existing observations, and we provide an approximate formula which expresses this evolution. Our models also suggest that the average tangential velocity within a bar drops by a factor of about 2 over approximately 45 initial rotation periods.