Bounds on Dark-Matter Annihilations from 21-cm Data

The observation of an absorption feature in the 21-cm spectrum at redshift $z\approx 17$ implies bounds on dark-matter (DM) annihilations for a broad range of masses, given that significant heating of the intergalactic medium would have erased such a feature. The resulting bounds on the DM annihilation cross sections are comparable to the strongest ones from all other observables.

Figure 1

Upper row: Bounds on DM annihilation cross sections in the instantaneous deposition approximation. The left panel shows bounds on the cross section times efficiency factor $f_{\mathrm{eff}}$ and fraction of annihilating DM $f_{\mathrm{DM}}^2$, obtained by demanding that the 21-cm absorption feature is not depleted from the value of standard cosmology ($-200\mathrm{mK}$ in our computation) down to $-100$ or $-50\mathrm{mK}$ because of DM heating. We take into account two different cosmological boost factors, and also show the weaker bound obtained by (unrealistically) ignoring DM clustering. The right panel shows bound on the cross section for a few main annihilation channels (bottoms, photons, muons, and electrons), using the $f_{\mathrm{eff}}$ described in the text, the mild boost factor of Eq. (7) and demanding that $T_{21}\gtrsim 100\mathrm{mK}$. Bottom row: Bounds on DM annihilation cross sections using delayed energy deposition and numerical primary spectra as described in the text. We demand that $T_{21}\gtrsim 100\mathrm{mK}$, showing the results for two different boost factors, as well as ignoring DM clustering. The left (right) panel shows DM decaying into bottom quarks or muons (photons or electrons).