  Astrophysical disks that are sufficiently cold and dense are linearly
unstable to the formation of axisymmetric rings as a result of the disk's
gravity. In practice, spiral structures are formed, which may in turn produce
bound fragments. We study a nonlinear dynamical path that can explain the
development of spirals in a local model of a gaseous disk on the subcritical
side of the gravitational instability bifurcation. Axisymmetric equilibria can
be radially periodic or localized, in the form of standing solitary waves. The
solitary solutions have an energy slightly larger than a smooth disk. They are
further unstable to non-axisymmetric perturbations with a wide range of
azimuthal wavenumbers. The solitary waves may act as a pathway to spirals and
fragmentation.
