  Novel "smectic-P" behavior, in which self-propelled particles form rows and
move on average along them, occurs generically within the
orientationally-ordered phase of simple models that we simulate. Both apolar
(head-tail symmetric) and polar (head-tail asymmetric) models with aligning and
repulsive interactions exhibit slow algebraic decay of smectic order with
system size up to some finite length scale, after which faster decay occurs. In
the apolar case, this scale is that of an undulation instability of the rows.
In the polar case, this instability is absent, but traveling fluctuations
disrupt the rows in large systems and motion and smectic order may
spontaneously globally rotate. These observations agree with a new hydrodynamic
theory which we present here. Variants of our models also exhibit active
smectic "A" and "C" order, with motion orthogonal and oblique to the layers
respectively.
