Black hole formation and growth with non-Gaussian primordial density perturbations

TL;DR

This study investigates how scale-dependent non-Gaussian primordial density fluctuations can facilitate the early formation and growth of supermassive black holes by increasing halo abundance and merger rates, compared to Gaussian fluctuations.

Contribution

It demonstrates through simulations that non-Gaussian fluctuations enhance black hole number density and mass growth, providing a potential explanation for early supermassive black holes.

Findings

Non-Gaussian fluctuations increase the total number density of black holes.

Non-Gaussian fluctuations lead to higher black hole masses and more massive black holes.

The fraction of halos forming black holes remains nearly unchanged.

Abstract

Quasars powered by massive black holes (BHs) with mass estimates above a billion solar masses have been identified at redshift 6 and beyond. The existence of such BHs requires almost continuous growth at the Eddington limit for their whole lifetime, of order of one billion years. In this paper, we explore the possibility that positively skewed scale-dependent non-Gaussian primordial fluctuations may ease the assembly of massive BHs. In particular, they produce more low-mass halos at high redshift, thus altering the production of metals and ultra-violet flux, believed to be important factors in BH formation. Additionally, a higher number of progenitors and of nearly equal-mass halo mergers would boost the mass increase provided by BH-BH mergers and merger-driven accretion. We use a set of two cosmological simulations, with either Gaussian or scale-dependent non-Gaussian primordial fluctuations to perform a proof-of-concept experiment to estimate how BH formation and growth are altered. We estimate the BH number density and the fraction of halos where BHs form, for both simulations and for two popular scenarios of BH formation (remnants of the first generation of stars and direct collapse in the absence of metals and molecular hydrogen). We find that the fractions of halos where BHs form are almost identical, but that non-Gaussian primordial perturbations increase the total number density of BHs for the both BH formation scenarios. We also evolve BHs using merger trees extracted from the simulations and find that non-Gaussianities increase both the BH masses and the number of the most massive BHs.