Abstract

Axion stars form in the centres of DM halos. If the axion is coupled to electromagnetism, then axion stars are unstable above a critical mass, and explode releasing low energy photons. Explosions are simulated in full (3+1)d numerical General Relativity. For axion masses in the range 1e-11 eV and couplings of order 1e-12 GeV^-1, axion star instability is predicted to happen in the first DM structures formed, with masses of order 1 Msol at redshifts z~30. The rate of such explosions is computed from halo merger histories in standard cosmology. Axion star explosions lead to enhanced DM decay during the dark ages, which heats the intergalactic medium. Constraints on anomalous heating from the CMB optical depth lead to new and powerful constraints on axion parameter space. Future 21cm measurements can provide complementary probes of axion parameter space to the DMRadio direct detection program.