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

TL;DR

This paper proposes that high-redshift galaxies called Little Red Dots originate from low-spin dark matter halos, explaining their abundance, compactness, and redshift distribution through a simple, observationally driven model.

Contribution

It introduces a novel low-spin halo model to explain LRD properties, aligning theoretical predictions with JWST observations without assuming specific powering mechanisms.

Findings

Low-spin halos account for LRD abundance and sizes.

Redshift evolution driven by surface brightness dimming and disk fraction.

Model matches observed clustering and spectral features.

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.