Late-Time Magnetogenesis Driven by Axionlike Particle Dark Matter and a Dark Photon

We propose a mechanism generating primordial magnetic fields after the e^{+}e^{-} annihilations. Our mechanism involves an ultralight axionlike particle (ALP) which constitutes the dark matter and a dark U(1)_{X} gauge boson introduced to bypass the obstacle placed by the conductivity of cosmic plasma. In our scheme, a coherently oscillating ALP amplifies the dark photon field, and part of the amplified dark photon field is concurrently converted to the ordinary magnetic field through the ALP-induced magnetic mixing. For the relevant ALP mass range 10^{-21}  eV≲m_{ϕ}≲10^{-17}  eV, our mechanism can generate B∼10^{-24}  G(m_{ϕ}/10^{-17}  eV)^{5/4} with a coherent length λ∼(m_{ϕ}/10^{-17}  eV)^{-1/2} kpc, which is large enough to provide a seed of the galactic magnetic fields. The mechanism also predicts a dark U(1)_{X} electromagnetic field E_{X}∼B_{X}∼80  nG(m_{ϕ}/10^{-17}  eV)^{-1/4}, which can result in interesting astrophysical or cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.