Particle decay and 21 cm absorption from first minihaloes

We consider the influence of decaying dark matter (DM) particles on the characteristics of 21 cm absorption in spectra of distant radio-loud sources - "21 cm forest" - from minihaloes with masses $M=10^5-10^7\msun$ virialized at $z_{vir} = 10$. We use 1D self-consistent hydrodynamic description to study evolution of minihaloes, and follow up their absorption characteristics from turnaround to virialization. We find that in the presence of decaying dark matter both thermal and dynamical evolution of minihaloes demonstrate significant deviation from those in the model without dark matter decay (standard recombination). We show that optical depth in 21 cm line is strongly suppressed in the presence of decaying particles: for $M=10^5-10^6\msun$ decaying dark matter with the energy rate deposited in baryonic gas $\xi_{L} = 0.59\times 10^{-25}$ s$^{-1}$ - the current upper limit of the energy deposit - decreases the optical depth and the equivalent width by an order of magnitude compared to the standard recombination. Thus additional ionization and heating from decaying DM particles almost "erases" absorption features from minihaloes with $M=10^5-10^6\msun$ for $\xi \simgt 0.3\xi_L$, which consequently considerably decreases the number of strong absorptions: for example, the number of absorptions with the equivalent width $W_\nu^{obs} \simgt 0.3$ kHz at $z\simeq10$ decreases more than 2.5 times for $\xi/\xi_{L} = 0.3$ and $\simgt$4.5 times for $\xi/\xi_{L} = 1$. We argue that "21 cm forest" absorptions might be a powerful probe of the presence of decaying dark matter in the early Universe.