Constraining Mixed Dark-Matter Scenarios of WIMPs and Primordial Black Holes from CMB and 21-cm observations

We consider the dark matter (DM) scenarios consisting of the mixture of WIMPs and PBHs and study how much fraction of the total DM can be PBHs. In such scenarios, PBHs can accrete the WIMPs and consequently enhance the heating and ionization in the intergalactic medium due to WIMP annihilations. We demonstrate that the CMB data can give the stringent bounds on the allowed PBH fraction which are comparable or even tighter than those from the gamma ray data depending on the DM masses. For instance, the MCMC likelihood analysis using the Planck CMB data leads to the bound on PBH DM fraction with respect to the total dark matter $f_{\rm PBH} \lesssim {\cal O}( 10^{-10}\sim 10^{-8})$ for the WIMP mass $m_{\chi}\sim {\cal O}(10\sim 10^3)$ GeV with the conventional DM annihilation cross section $\langle \sigma v \rangle=3 \times 10^{-26}~\rm cm^3/s $. We also investigate the feasibility of the global 21-cm signal measurement to provide the stringent constraints on the PBH fraction.