  We experimentally study universal nonequilibrium dynamics of two-dimensional
atomic Bose gases quenched from repulsive to attractive interactions. We
observe the manifestation of modulational instability that, instead of causing
collapse, fragments a large two-dimensional superfluid into multiple wave
packets universally around a threshold atom number necessary for the formation
of Townes solitons. We confirm that the density distributions of quench-induced
solitary waves are in excellent agreement with the stationary Townes profiles.
Furthermore, our density measurements in the space and time domain reveal
detailed information about this dynamical process, from the hyperbolic growth
of density waves, the formation of solitons, to the subsequent collision and
collapse dynamics, demonstrating multiple universal behaviors in an attractive
many-body system in association with the formation of a quasistationary state.
