Primordial Intermediate and Supermassive Black Hole formation during the electron-positron annihilation epoch

Abstract

Some of the Intermediate Mass Black Hole (IMBH) candidates observed at the center of galaxies or in globular clusters and some of the Supermassive Black Holes (SMBHs) seen at the center of many galaxies might be of primordial origin. Indeed, Primordial Black Holes (PBHs) of such mass could have formed when the Universe was $\sim$1-10$^3$ s old, due to the collapse of density fluctuations. In particular, when the Universe was $\sim 1$ 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 $5\times 10^{3}$ - $5\times 10^{8}M_{\odot}$ PBHs with a density as high as $\sim 10^{10}$/Gpc$^3$. 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).