A cosmologist's take on Little Red Dots

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 $μ$-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.

Figures (5)

• Figure 1: Constraints on the parameter space of PBH formation. The upper panel shows limits on the amplitude of the primordial power spectrum, in terms of the high-redshift PBH mass $M_\text{\tiny PBH}$, while the lower panel provides the current constraints on the PBH abundance for a monochromatic population (see the main text for a description of the plotted bounds). The colored lines in the upper panel indicate different values of the PBH abundance, while the light-blue band in the lower panel shows the number density of observed LRDs.
• Figure 2: Number density of environments which are able to form a $10^7 M_\odot$ PBH through hierarchical mergers of lighter $10^3 M_\odot$ seeds, as a function of redshifts $z$. Red and blue colors correspond to mergers proceeding through the democratic and oligarchic scenarios, respectively. Solid, dashed and dotted (shown in black as both cases overlap) lines indicate different values of the PBH abundance $f_\text{\tiny PBH}$, while the bands consider values of the NFW concentration parameter within the range $c_\text{\tiny DM} = (0.1 - 1)$. The black rectangle indicates the parameter space of the LRDs observed so far Dayal:2024zwq. BHL and PR corresponds to assuming the maximum abundance allowed by those constraints at the assumed initial mass $10^3 M_\odot$, see Fig. \ref{['fig:direct']}.
• Figure 3: Parameter space of the accretion-metallicity model, defined by the fragmentation and accretion coefficients, $f_\text{\tiny frag}$ and $\lambda$, required to reproduce QSO1 within its $1\sigma$ confidence intervals, $M_\text{\tiny QSO1} = 10^{7.7} \, M_\odot$ and $Z_\text{\tiny QSO1} = 10^{-2.08}Z_\odot$. The orange and blue contours correspond to different choices of the ejection parameter $f_\text{\tiny out}$. The inset shows the redshift evolution of the PBH and stellar masses corresponding to the yellow star, that marks the central values of the QSO1 parameters. The dashed purple line indicates the maximum $f_{\text{\tiny frag}}$ allowed within $3\sigma$ of the observed metallicity. The gray region shows the parameter space where the metal enrichment from structure formation could contribute to the metallicity of QSO1.
• Figure 4: Behavior of the hierarchical merger probability as a function of the environment escape velocity. Solid lines show the numerical results, while dashed lines indicate the analytical fit using the sigmoid function presented in Eq. \ref{['fitepsilon']} of the main text.
• Figure 5: Halo mass function at various redshifts of interest for this work.