The 21 cm signal and the interplay between dark matter annihilations and astrophysical processes

TL;DR

This paper investigates how dark matter annihilations influence the 21 cm signal during cosmic reionization, emphasizing the challenges in distinguishing DM effects from astrophysical processes and identifying specific DM mass signatures.

Contribution

The study provides updated modeling of DM annihilation effects on the 21 cm signal, considering recent cosmological constraints and halo contributions, highlighting the difficulty in isolating DM signatures.

Findings

DM annihilations can affect the 21 cm power spectrum.

Only DM particles around 100 MeV produce clear signals.

Degeneracies with astrophysical effects complicate detection.

Abstract

Future dedicated radio interferometers, including HERA and SKA, are very promising tools that aim to study the epoch of reionization and beyond via measurements of the 21 cm signal from neutral hydrogen. Dark matter (DM) annihilations into charged particles change the thermal history of the Universe and, as a consequence, affect the 21 cm signal. Accurately predicting the effect of DM strongly relies on the modeling of annihilations inside halos. In this work, we use up-to-date computations of the energy deposition rates by the products from DM annihilations, a proper treatment of the contribution from DM annihilations in halos, as well as values of the annihilation cross section allowed by the most recent cosmological measurements from the Planck satellite. Given current uncertainties on the description of the astrophysical processes driving the epochs of reionization, X-ray heating and Lyman-$\alpha$ pumping, we find that disentangling DM signatures from purely astrophysical effects, related to early-time star formation processes or late-time galaxy X-ray emissions, will be a challenging task. We conclude that only annihilations of DM particles with masses of $\sim100$ MeV, could leave an unambiguous imprint on the 21 cm signal and, in particular, on the 21 cm power spectrum. This is in contrast to previous, more optimistic results in the literature, which have claimed that strong signatures might also be present even for much higher DM masses. Additional measurements of the 21 cm signal at different cosmic epochs will be crucial in order to break the strong parameter degeneracies between DM annihilations and astrophysical effects and undoubtedly single out a DM imprint for masses different from $\sim100$ MeV.