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 oscillation. The dark photons with a net helicity lead to a metric vorticity and can generate chiral substructures. For axion masses in the range $10^{ -28} \, \mathrm{ eV} \lesssim m_a \lesssim 10^{ -22} \, \mathrm{ eV}$, the resulting inhomogeneities collapse to form halos with masses spanning $M_{ \rm halo} \sim 10^5 \, M_{ \odot}$ to $10^{ 11} \, M_{ \odot}$, with halo sizes ranging from $O(1)$ to $O(10^{ 6}) \, \mathrm{ pc}$. During halo collapse, the induced vorticity could mediate efficient angular-momentum transport, which enables monolithic collapse and provides primordial seeds for the early formation of supermassive black holes.
Categories: gr-qc astro-ph.CO hep-ph