A cosmologist's take on Little Red Dots

TL;DR

This paper explores the origins of the Little Red Dots observed by JWST, assessing primordial black holes and hierarchical mergers as potential sources, and evaluates their growth via accretion within cosmological constraints.

Contribution

It systematically analyzes primordial black hole formation, hierarchical merger scenarios, and gas accretion models to explain the properties of high-redshift compact sources.

Findings

Direct primordial BH formation is excluded by CMB constraints.

Hierarchical mergers face challenges due to rare high-redshift halos.

Gas accretion onto PBHs could reproduce observed properties under certain conditions.

Abstract

The James Webb Space Telescope (JWST) has uncovered a population of compact, high-redshift sources, the Little Red Dots (LRDs), which may host supermassive black holes (BHs) significantly heavier than their stellar content compared with local scaling relations. These objects challenge standard models of early galaxy formation and may represent an extreme class of early BH hosts. In this paper, we investigate whether these BHs could have a primordial origin. We first show that the direct formation of these BH masses in the early Universe is excluded by stringent CMB $\mu$-distortion limits. We then investigate the assembly of massive BHs from lighter, observationally allowed primordial black holes (PBHs) via hierarchical mergers, finding that, although this channel can operate depending on the merger history, it faces challenges in explaining the observations due to the rarity of the required high-redshift dark matter halos. Finally, we estimate gas accretion onto intermediate-mass PBHs, while jointly tracking metallicity evolution, and identify regions of parameter space in which such growth could reproduce the observed properties of LRDs. As a special case, we focus on the strongly lensed source QSO1, whose extremely low metallicity and large mass provide a stringent test of these formation channels.