  We examine the influence of quenched disorder on the flocking transition of
dense polar active matter. We consider incompressible systems of active
particles with aligning interactions under the effect of either quenched random
forces or random dilution. The system displays a continuous disorder-order
(flocking) transition, and the associated scaling behavior is described by a
new universality class which is controlled by a quenched Navier-Stokes fixed
point. We determine the critical exponents through a perturbative
renormalization group analysis. We show that the two forms of quenched
disorder, random force and random mass (dilution), belong to the same
universality class, in contrast with the situation at equilibrium.
