Multiarm spirals on the periphery of disc galaxies
  Spiral patterns in some disc galaxies have two arms in the centre, and three
or more arms on the periphery. The same result is also obtained in numerical
simulations of stellar and gaseous discs. We argue that such patterns may occur
due to fast cooling of the gas, resulting in formation of giant molecular
clouds. The timescale of this process is 50 Myr, the factor of 10 shorter than
of ordinary secular instability. The giant molecular clouds give rise to
multiarm spirals through the mechanism of swing amplification.
