  We show that incompressible polar active fluids can exhibit an ordered,
coherently moving phase even in the presence of quenched disorder in two
dimensions. Unlike such active fluids with annealed (i.e., time-dependent)
disorder only, which behave like equilibrium ferromagnets with long-range
interactions, this robustness against quenched disorder is a fundamentally
non-equilibrium phenomenon. The ordered state belongs to a new universality
class, whose scaling laws we calculate using three different renormalization
group schemes, which all give scaling exponents within 0.02 of each other,
indicating that our results are quite accurate. Our predictions can be
quantitatively tested in readily available artificial active systems, and imply
that biological systems such as cell layers can move coherently in vivo, where
disorder is inevitable.
