  We study collections of self-propelled rods (SPR) moving in two dimensions
for packing fractions less than or equal to 0.3. We find that in the
thermodynamical limit the SPR undergo a phase transition between a disordered
gas and a novel phase-separated system state. Interestingly, (global)
orientational order patterns -- contrary to what has been suggested -- vanish
in this limit. In the found novel state, the SPR self-organize into a highly
dynamical, high-density, compact region - which we call aggregate - which is
surrounded by a disordered gas. Active stresses build inside aggregates as
result of the combined effect of local orientational order and active forces.
This leads to the most distinctive feature of these aggregates: constant
ejection of polar clusters of SPR. This novel phase-separated state represents
a novel state of matter characterized by large fluctuations in volume and
shape, related to mass ejection, and exhibits positional as well as
orientational local order. SPR systems display new physics unseen in other
active matter systems due to the coupling between density, active stresses, and
orientational order (such coupling cannot be reduced simply to a coupling
between speed and density).
