Signatures of particle decay in 21 cm absorptions from first minihalos

TL;DR

This study investigates how decaying dark matter influences 21 cm absorption signals from minihalos, revealing that even modest decay rates can significantly diminish observable features, especially at high redshifts.

Contribution

It provides a self-consistent hydrodynamic model showing the impact of dark matter decay on 21 cm signals from minihalos in the early universe.

Findings

Decaying dark matter reduces 21 cm optical depth in minihalos.

Modest ionization rates can erase absorption features.

21 cm observations can constrain dark matter decay rates at high redshifts.

Abstract

The imprint of decaying dark matter (DM) particles in characteristics of the "21 cm fores" -- absorptions in 21 cm from minihalos in spectra of distant radio-loud sources -- is considered within a 1D self-consistent hydrodynamic description of minihalos from their turnaround point to virialization. The most pronounced influence of decaying DM on evolution of minihalos is found in the mass range $M=10^5-10^6\msun$, for which unstable DM with the current upper limit of the ionization rate $\xi_{L} = 0.59\times 10^{-25}$ s$^{-1}$ depresses 21 cm optical depth by an order of magnitude compared to the standard recombination scenario. Even rather a modest ionization $\xi \sim 0.3\xi_L$ practically "erases" absorption features and results in a considerable decrease (by factor of more than 2.5) of the number of strong ($W_\nu^{obs} \simgt 0.3$ kHz at $z\simeq 10$) absorptions. At such circumstances broad-band observations are to be more suitable for inferring physical conditions of the absorbing gas. X-ray photons from stellar activity of initial episodes of star formation can compete the contribution from decaying DM only at $z<10$. Therefore, when observed the 21 cm signal will allow to follow evolution of decaying DM particles in the redshift range $z=10-15$. Contrary, in case of non-detection of the 21 cm signal in the frequency range $\nu<140$ MHz a lower limit on the ionization rate from decaying dark matter can be established.