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

Abstract

The Little Red Dots (LRDs) are high-redshift galaxies uncovered by JWST, characterized by small effective radii ($R_{\mathrm{eff}}\sim 80-300$ 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 $z\sim 5$ 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.

Accepted for publication in The Astrophysical Journal Letters. 12 pages, 5 figures

References in corpus (30)

• The Structure of Cold Dark Matter Halos
• The Average Star Formation Histories of Galaxies in Dark Matter Halos from z=0-8
• The Formation of Galactic Disks
• Galactic star formation and accretion histories from matching galaxies to dark matter haloes
• In the Beginning: The First Sources of Light and the Reionization of the Universe
• The Structure of Dark Matter Haloes in Hierarchical Clustering Models
• Compaction and Quenching of High-z Galaxies in Cosmological Simulations: Blue and Red Nuggets
• A population of red candidate massive galaxies ~600 Myr after the Big Bang
• New Determinations of the UV Luminosity Functions from z~9 to z~2 show a remarkable consistency with halo growth and a constant star formation efficiency
• NIRCam Performance on JWST In Flight
• The Confinement of Star-Forming Galaxies into a Main Sequence through Episodes of Gas Compaction, Depletion, and Replenishment
• JADES. The diverse population of infant Black Holes at 4<z<11: merging, tiny, poor, but mighty
• A Revised Model for the Formation of Disk Galaxies: Low Spin and Dark-Halo Expansion
• Spectroscopic confirmation of two luminous galaxies at $z\sim 14$
• JWST CEERS & JADES Active Galaxies at z = 4-7 Violate the Local $M_{\bullet }-M_{\star }$ Relation at $>3\sigma$: Implications for Low-Mass Black Holes and Seeding Models
• Origin of Quasar Progenitors from the Collapse of Low-Spin Cosmological Perturbations
• A Maximum Stellar Surface Density in Dense Stellar Systems
• JWST UNCOVER: Extremely red and compact object at$z_{\mathrm{phot}}\simeq 7.6$ triply imaged by Abell 2744
• On the spin parameter of dark-matter haloes
• The specific angular momentum of disc galaxies and its connection with galaxy morphology, bar structure and disc gravitational instability
• Mildly Super-Eddington Accretion Onto Slowly-Spinning Black Holes Explains the X-Ray Weakness of the Little Red Dots
• Exponential Galaxy Disks from Stellar Scattering
• The Faint End of the Quasar Luminosity Function at $z\sim 5$ from the Subaru Hyper Suprime-Cam Survey
• The Redshift Evolution of the $M_{\bullet }-M_{\star }$ Relation for JWST's Supermassive Black Holes at $z>4$
• From lenticulars to blue compact dwarfs: the stellar mass fraction is regulated by disc gravitational instability
• Simulations predict intermediate-mass black hole formation in globular clusters
• Exploring the AGN Fraction of a Sample of JWST's Little Red Dots at $5<z<8$: Overmassive Black Holes Are Strongly Favored
• Sizes and Stellar Masses of the Little Red Dots Imply Immense Stellar Densities
• The influence of initial mass segregation on the runaway merging of stars
• Little Red Dots from Low-Spin Galaxies at High Redshifts

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• Investigating the Growth of Little Red Dot Descendants at z<4 with the JWST
• Little Red Dots as Hidden Neutrino Sources
• The implications of overmassive black holes at $z>5$ for quasar and black hole growth
• Islands of Electromagnetic Tranquility in Our Galactic core and Little Red Dots that Shelter Molecules and Prebiotic Chemistry