A variant on the mechanism for bar instabilities

The problem of how disc galaxies avoid forming bars remains unsolved. Many galaxy models having reasonable properties continue to manifest vigorous instabilities that rapidly form strong bars and no widely-accepted idea has yet been advanced to account for why this does not generally happen in most disc galaxies. The unstable mode that is responsible for bar formation is believed to be a standing wave in a cavity that reflects off the disc centre and the corotation radius, with amplification at corotation. Here we use simulations to address one further idea that may perhaps inhibit the feedback loop and therefore contribute to stability, which is to make the disc centre dynamically hot and/or to taper away mass from the inner disc, which could be masked by a bulge. Unfortunately, we find that neither strategy makes much difference to the global stability of the disc in the models we have tried. We did, however, discover one interesting wrinkle to the cavity mode: in strongly cutout discs, the wave may reflect from a radius in the disc that is clearly outside the centre, although the ingoing trailing wave still reflects into an outwardly-travelling, leading wave.