Cosmic Outliers: Low-Spin Halos Explain the Abundance, Compactness, and Redshift Evolution of the Little Red Dots

Fabio Pacucci, Abraham Loeb

First listed 2025-06-03 | Last updated 2025-07-07

Abstract

The Little Red Dots (LRDs) are high-redshift galaxies uncovered by JWST, characterized by small effective radii ($R_{\rm eff} \sim 80-300$ pc), number densities that are intermediate between those of typical galaxies and quasars, and a redshift distribution peaked at $z \sim 5$. We present a theoretical model in which the LRDs descend from dark matter halos in the extreme low-spin tail of the angular momentum distribution. Within this framework, we explain their three key observational signatures: (i) abundance, (ii) compactness, and (iii) redshift distribution. Our model focuses on observed, not modeled, properties; it is thus independent of whether they are powered primarily by a black hole or stars. We find that the assumption that the prototypical LRD at $z\sim5$ originates from halos in the lowest $\sim 1\%$ of the spin distribution is sufficient to reproduce both their observed number densities and physical sizes. The redshift evolution of their observability is driven by the interplay between the evolving compact disk fraction and cosmological surface brightness dimming. This effect leads to a well-defined "LRDs Era" at $4<z<8$, during which the LRDs are common and detectable; at $z<4$, they are bright but rare, while at $z>8$, they are common but faint. Finally, we test the predicted redshift trend against observational data, finding excellent agreement. Additional observational support comes from their excess small-scale clustering and spectral signatures of extreme core densities, both of which are expected outcomes of galaxy formation in low-spin halos. These findings suggest that the LRDs are not a fundamentally distinct population but the natural manifestation of galaxies forming in the rarest, lowest angular momentum environments.

Short digest

Proposes that Little Red Dots are the visible outcomes of galaxies forming in the extreme low-spin tail of the halo angular-momentum distribution, using Mo–Mao–White scaling to link halo spin to compact disk sizes. Assuming progenitors in the lowest ~1% of spins at z ~ 5 reproduces both the observed number densities and compact effective radii of ~80–300 pc, without requiring a choice between BH- or star-dominated power. The observed redshift trend arises from the competition between an increasing compact-halo fraction and cosmological surface-brightness dimming, yielding an “LRDs Era” at 4 < z < 8; at z < 4 they are bright but rare, while at z > 8 they are common but faint. Agreement with current counts, plus excess small-scale clustering and spectral signs of extreme core densities, supports the low-spin origin.

Key figures to inspect

• Figure 1: Check where LRD number densities sit between LBGs and UV-selected quasars; this sets the normalization the low-spin model must match.
• Figure 2: Inspect the lognormal spin PDF and the critical spin λcrit; note that λcrit lies in the lowest ~1% tail, anchoring the rarity needed for LRD abundances and compactness.
• Figure 3: Compare the predicted number density versus Reff at z ~ 5 across λ; verify that the observed Reff ~80–300 pc band intersects the observed density range only for very low λ, quantifying the required tail.
• Figure 4: Trace the redshift-dependent compact fraction against the JWST/NIRCam surface-brightness limit (~25.2 mag/arcsec^2); the overlap explains the “LRDs Era” at 4 < z < 8 and why systems are bright-but-rare at z < 4 and common-but-faint at z > 8.