2024MNRAS.531L..40S

Some of the Intermediate Mass Black Hole candidates observed at the centre of galaxies or in globular clusters and some of the Supermassive Black Holes seen at the centre of many galaxies might be of primordial origin. Indeed, Primordial Black Holes (PBHs) of such mass could have formed when the Universe was ∼1-10³ s old, due to the collapse of density fluctuations. In particular, when the Universe was ∼1 s in age, Electron-Positron Annihilation (EPA) took place. We explore the formation of intermediate mass and supermassive PBHs, taking into account the effect of the EPA when the fluctuations have a running tilt power-law spectrum: when these cross the 10^–0.5-10^3.0 s Universe horizon they could produce 5 × 10³-5 × 10⁸ M_⊙ PBHs with a density as high as ∼10¹⁰/Gpc³. On average, this implies a population of about one thousand PBHs in the Local Group of Galaxies, with the nearest one at about 250 kpc, just under half the distance to the Andromeda galaxy (M31).