Signatures of inhomogeneous dark matter annihilation on 21-cm

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 $\left<σv\right>/m_χ\sim 10^{-27} {\rm cm^3 s^{-1} GeV^{-1}}$, 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.

💡 Research Summary

The paper investigates how dark‑matter (DM) annihilation, which injects energy into the intergalactic medium (IGM), modifies the cosmological 21‑cm signal when the annihilation rate is allowed to be spatially inhomogeneous. Because the annihilation rate scales as the square of the DM density, the formation of halos and sub‑halos during cosmic dawn dramatically boosts the local energy injection compared with a uniform background. The authors develop a semi‑analytic framework that couples a halo‑based boost factor to the widely used 21cmFAST code, thereby self‑consistently evolving the gas temperature, ionization fraction, and Lyman‑α background in the presence of DM‑induced heating and ionization.

Key components of the model are: (i) a calculation of the annihilation luminosity inside each halo by integrating the NFW density profile, (ii) a conditional halo mass function (based on Press‑Schechter theory, normalized to the Sheth‑Tormen fit) that depends on the local overdensity δ, and (iii) a “boost factor” B(x) that combines the contributions from collapsed halos and the smooth IGM. The boost factor is defined as the ratio of the total (halo + IGM) injection rate to the homogeneous injection rate, and it explicitly contains the term