Ultradense dark matter haloes accompany primordial black holes

Abstract

ABSTRACT Primordial black holes (PBHs) form from large-amplitude initial density fluctuations and may comprise some or all of the dark matter. If PBHs have a broadly extended mass spectrum, or in mixed PBH-particle dark matter scenarios, the extreme density fluctuations necessary to produce PBHs also lead to the formation of a much greater abundance of dark matter minihaloes that form during the radiation epoch with internal densities potentially of the order of 1012 M⊙ pc−3. We develop an analytical description of the formation of these ultradense haloes and use it to quantitatively compare PBH and halo distributions. PBHs that contribute only a per cent level fraction of the dark matter are accompanied by ultradense haloes that nevertheless comprise an order–unity fraction. These haloes would consist of either particle dark matter or much smaller PBHs. This finding significantly alters the predictions of many PBH scenarios, enabling a variety of new observational tests.