Signatures of inhomogeneous dark matter annihilation on 21-cm

Abstract

The energy released from dark matter (DM) annihilation leads to additional ionization and heating of the intergalactic gas, impacting the hydrogen 21-cm signal during the cosmic dawn. The dark matter annihilation rate scales with its density squared and becomes inhomogeneously boosted with structure formation. This paper examines the inhomogeneity in DM annihilation rate induced by the growth of DM halo structures, and we show that this effect can significantly amplify the spatial fluctuations in temperature and ionization fraction of the gas. Consequently, the fluctuations in the 21-cm brightness temperature may also be enhanced. We showcase these effects for a DM mass of 100 MeV annihilating into $\rm{e}^-\rm{e}^+$ at a rate of $\left<\sigma v\right>/m_\chi \sim 10^{-27} {\rm cm^3 s^{-1} GeV^{-1}}$, which is consistent with current constraints set by the cosmic microwave background. We find that, compared to the homogeneous calculations, inhomogeneous annihilation can enhance the 21-cm power spectrum by up to a factor of 130 over the scales of $k \in [0.05, 3]\ {\rm{Mpc^{-1}}}$ at redshifts $11-16$. Such signatures could potentially be detected by upcoming radio observatories such as the Square Kilometer Array telescope.