Excess of substructure due to primordial black holes

¹ Facultad de Matemática, Astronomía, Física y Computación, UNC, Córdoba, Argentina
² Instituto de Astronomía Teórica y Experimental, CONICET–UNC, Córdoba, Argentina
³ Observatorio Astronómico de Córdoba, UNC, Córdoba, Argentina

^⋆ Corresponding author: patricio.colazo@mi.unc.edu.ar

Received: 19 May 2025
Accepted: 31 July 2025

Abstract

Context. In this paper we explore the impact of primordial black holes (PBHs) on the abundance of low mass haloes and subhaloes in the dark and low stellar mass regime, and examine how these effects can be measured through fluctuations in strong lensing and brightness fluctuations in clusters of galaxies, providing potential ways to constrain the fraction of dark matter in PBHs.

Aims. Various dark matter candidates leave unique imprints on the low mass range of the halo mass function that can be challenging to detect. Among these are the hot and warm dark matter models that predict a reduced abundance of low mass structures compared to the cold dark matter with a cosmological constant (ΛCDM) model. Models with PBHs also affect this mass range, but in the opposite direction, producing an increase in these low mass objects. By examining lensing perturbations in galaxy clusters, constraints can be placed on the low mass subhalo abundance and, therefore, on these different models for dark matter. We aim to provide predictions useful for this type of perturbations for the PBH case. Additionally, we examine the abundance of haloes and subhaloes in the range where the stellar mass to halo mass relation is steeply increases, which could be compared to brightness fluctuations in clusters of galaxies due to low mass satellites with low luminosities.

Methods. We ran cosmological simulations using the SWIFT code, comparing a fiducial model with alternative inflationary models both with and without PBHs.

Results. We find a significant excess of substructure in the presence of PBHs compared to the ΛCDM model, without altering the abundance of high mass haloes at redshift zero. This increase is of up to a factor of six for extended PBH mass functions with an exponential cut-off at M_PBH = 10² M_⊙ in the range of parameter space where they could make up all of the dark matter. Similar increases are also produced when this fraction is smaller, even at sub-percent levels, for PBHs that show an exponential cut-off in their mass function at masses M_PBH = 10⁴ M_⊙.