Probing Small-scale Dark Matter Clumping with the Large-scale 21 cm Power Spectrum

Abstract The 21 cm line of hydrogen is the most promising probe of the Dark Ages and Cosmic Dawn. We combine hydrodynamical simulations with a large-scale grid in order to calculate the effect of nonlinear structure formation on the large-scale 21 cm power spectrum, focusing on redshifts z = 20–40. As the clumping effect arises from small-scale density fluctuations, it offers a unique opportunity to probe the standard cold dark matter model in a new regime and thus potentially investigate the properties of dark matter. To this end, we also study a warm dark matter–like model with a Gaussian cutoff on a scale of 50 kpc. We find that clumping has a significant impact on the large-scale 21 cm power spectrum, requiring a substantial correction to standard theoretical predictions. For example, for the Dark Ages case at z = 30 and wavenumber k = 0.05 Mpc −1 , small-scale clustering enhances the 21 cm power spectrum by 13%. Once Ly α coupling kicks in due to the first stars, the 21 cm signal strengthens, and the effect of clumping grows; it suppresses the observable power spectrum at z = 20 by 45%, while the warm dark matter–like model has less than half the clumping impact. The clumping effect is significantly higher than the sensitivity of the planned Square Kilometre Array AA ⋆ configuration, by up to a factor of 20 for standard cold dark matter, though detection will require separation from foregrounds and from astrophysical contributions to the 21 cm power spectrum.