Lyman-α Forest Constraints on Primordial Black Holes as Dark Matter

The renewed interest in the possibility that primordial black holes (PBHs) may constitute a significant part of dark matter has provided motivation for revisiting old observational constraints, as well as developing new ones. We present new limits on the PBH abundance, from a comprehensive analysis of high-resolution high-redshift Lyman-α forest data. Poisson fluctuations in the PBH number density induce a small-scale power enhancement which departs from the standard cold dark matter prediction. Using a grid of hydrodynamic simulations exploring different values of astrophysical parameters, we obtain a marginalized upper limit on the PBH mass of f_{PBH}M_{PBH}∼60M_{⊙} at 2σ, when a Gaussian prior on the reionization redshift is imposed, preventing its posterior distribution from peaking on very high values, which are disfavored by the most recent estimates obtained both through cosmic microwave background and intergalactic medium observations. Such a bound weakens to f_{PBH}M_{PBH}∼170M_{⊙} when a conservative flat prior is instead assumed. Both limits significantly improve on previous constraints from the same physical observable. We also extend our predictions to nonmonochromatic PBH mass distributions, ruling out large regions of the parameter space for some of the most viable PBH extended mass functions.