Signatures of particle decay in 21 cm absorptions from first minihalos

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 $M={10}^5-{10}^6\text{msun}$, 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 ($W_{\nu }^{obs}\text{simgt}0.3$ 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.