The Lifetimes of Spiral Patterns in Disc Galaxies
  The rate of internally-driven evolution of galaxy discs is strongly affected
by the lifetimes of the spiral patterns they support. Evolution is much faster
if the spiral patterns are recurrent short-lived transients rather than
long-lived, quasi-steady features. As rival theories are still advocated based
on these two distinct hypotheses, I review the evidence that bears on the
question of the lifetimes of spiral patterns in galaxies. Observational
evidence from external galaxies is frustratingly inconclusive, but the velocity
distribution in the solar neighbourhood is more consistent with the transient
picture. I present simulations of galaxy models that have been proposed to
support quasi-steady, two-arm spiral modes that in fact evolve quickly due to
multi-arm instabilities. I also show that all simulations to date manifest
short-lived patterns, despite claims to the contrary. Thus the transient
hypothesis is favoured by both numerical results and the velocity distribution
in the solar neighbourhood.
