Primordial black holes in the Dark Ages: Observational prospects for future 21cm surveys

We consider the signatures of a population of primordial black holes (PBHs) in future observations of 21cm radiation from neutral hydrogen at high redshift. We focus on PBHs in the mass range $5 \times 10^{10} kg \lesssim M_{PBH} \lesssim 10^{14} kg$, which primarily influence the intergalactic medium (IGM) by heating from direct Hawking radiation. Our computation takes into account the black hole graybody factors and the detailed energy dependence of photon and e+/- absorption by the IGM. We find that for black holes with initial masses between $5 \times 10^{11} kg \lesssim M_{PBH} \lesssim 10^{14} kg$, the signal mimics that of a decaying dark matter species. For black holes in the range $5 \times 10^{10} kg \lesssim M_{PBH} \lesssim 5 \times 10^{11} kg$, the late stages of evaporation produce a characteristic feature in the 21cm brightness temperature that provides a unique signature of the black hole population. If no signal is observed, then 21cm observations will provide significantly better constraints on PBHs in the mass range $5 \times 10^{10} kg \lesssim M_{PBH} \lesssim 10^{12} kg$ than are currently available from the diffuse $\gamma$-ray background.