Dynamical Heating from Dark Compact Objects and Axion Minihalos: Implications for the 21-cm Signal

Abstract

The temperature of baryons at the end of the cosmic dark ages can be inferred from observations of the 21-cm hyperfine transition in neutral hydrogen. Any energy injection from the dark sector can therefore be detected through these measurements. Dark compact objects and dark-matter substructures can modify the baryon temperature by transferring heat via dynamical friction. In this work, we evaluate the prospects for detecting dynamical friction-induced heating from dark compact objects with a mass in the range $10^2 M_{\odot}$ to $10^5 M_{\odot}$, as well as from axion minihalos, using upcoming 21-cm experiments. We find that both the 21-cm global signal and power-spectrum measurements will be sensitive to dark compact objects that constitute about 10% of the dark matter, and will substantially improve our sensitivity to axion-like particles with masses in the range $10^{-18}$ eV to $10^{-9}$ eV.

Figures (2)

• Figure 1: Left: Baryon temperature $T_{\rm B}$ as a function of redshift for MACHOs of mass $M = 10^2 \, \rm M_{\odot}$ (olive) and $M = 10^4 \, \rm M_{\odot}$ (red), and for axion minihalos with an initial mass $M_{\rm h,c} = 10^{-1} \, \rm M_{\odot}$ (blue), assuming MACHOs/axions constitute 100% of the DM. The solid (dashed) lines correspond to a DM-baryon relative velocity at recombination of $v_{\rm rel}^* =30 \, (50) km\per s$. Middle: The sky-averaged 21-cm brightness temperature $\langle T_{21} \rangle(z)$. Right: The 21-cm power spectrum $\Delta_{21}^2$ as a function of comoving wavenumber at $z = 17$. The golden arrow indicates the expected HERA sensitivity of 92.7mK after $10^3$ hours of integration time, following Refs. Munoz:2018jwqBarkana:2022hko.
• Figure 2: Left: Forecasted sensitivity (in red) of the 21-cm signal to the MACHO fraction as a function of MACHO mass. The solid curve marks the region in which DF heating yields a brightness temperature that is 50 mK higher than expected in $\Lambda$CDM, while the dashed line corresponds to the forecasted sensitivity from HERA. The other limits come from the observed half-light radius of Ultrafaint dwarf galaxies (UFDs) Graham:2023unf, the existence of wide-binaries Ramirez:2022mys, non-detection of lensing effects of compact radio sources Zhou:2021tvp, and the caustic crossing of Icarus Oguri:2017ock. See also Refs. Wadekar:2022ymqKim:2025gck for bounds under different assumptions of the MACHO density profile. Right: Forecasted sensitivity (in red) of the 21-cm signal to ALP symmetry breaking scale $f_a$ for ALP masses $m_a$. The solid and dashed curves correspond to an axion minihalo with an initial mass of $M_{\rm h,c} = 0.015 \, \rm M_{\odot}$. Also shown are bounds derived from astrophysical probes of the axion-photon coupling assuming $g_{a\gamma\gamma} = \alpha_{\rm EM} / (2 \pi f_a)$Reynes:2021bpeNing:2024ekyMarsh:2017yvcMuller:2023vjmFermi-LAT:2016nkzReynolds:2019uqt, cosmology (in purple) Harigaya:2025poxMurgia:2019duy, and the superradiance bounds (in green) taken from Refs. Mehta:2020kwuBaryakhtar:2020gaoUnal:2020jiy2025MNRAS.tmp.1518HWitte:2024drg, as compiled in Ref. AxionLimits.