Parity-violating dark photon halos

Abstract We propose a mechanism for the generation of gravitationally bound dark photon halos during the matter-dominated era. Coupled to an ultralight axion field through a parity-violating Chern-Simons term, dark photons can be produced by the tachyonic instability of axion coherent oscillations. The dark photons with a net helicity lead to a metric vorticity and can generate chiral substructures. We propose that at early times, the axion is held at a fixed value before being released at a characteristic redshift z * , at which the field begins coherent oscillations in an approximately quadratic potential. For parameters that satisfy the pre-trap-release energy budget, the resulting halos span masses approximately 10 6 -10 7 , M ⊙ and radii ranging from ∼ 10 2 kpc to a few ∼ Mpc. During subsequent nonlinear evolution, the induced vorticity could mediate angular-momentum transport and influence collapse dynamics, potentially contributing to early structure formation and black hole seeding.