  We study a continuum model of overdamped self-propelled particles with an
aligning interaction in two dimensions. By combining analytical and numerical
work, we map out the phase diagram for generic parameters. We find that the
system self-organizes into two robust structures in different regions of
parameter space: solitary waves of ordered swarms moving through a low density
disordered background, and stationary asters. The self-regulating nature of the
flow yields phase separation, ubiquitous in this class of systems, and controls
the formation of solitary waves. Self-propulsion and the associated active
convection play a crucial role in aster formation. A new result of our work is
a phase diagram that displays these different regimes in a unified manner.
