Connecting the Dots: UV-Bright Companions of Little Red Dots as Lyman-Werner Sources Enabling Direct Collapse Black Hole Formation

Josephine F. W. Baggen, Matthew T. Scoggins, Pieter van Dokkum, Zoltán Haiman, Alberto Torralba, Jorryt Matthee

First listed 2026-02-02 | Last updated 2026-02-05

Abstract

We compile a sample of 83 Little Red Dots (LRDs) with JWST imaging and find that a substantial fraction ($\sim$43%, rising to $\gtrsim$85% for the most luminous LRDs) host one or more spatially offset, UV-bright companions at projected separations of $0.5\rm \, kpc \lesssim d\lesssim 5 \rm \,kpc$, with median of $\langle d \rangle = 1.0\,\mathrm{kpc}$. This fraction is even higher when smaller spatial scales are probed at high S/N ratio: we show that the two most strongly lensed LRDs known to date, A383-LRD and the newly discovered A68-LRD, both have UV-bright companions at separations of only $d\sim0.3$ kpc, below the resolution limit of most unlensed JWST samples. We explore whether these ubiquitous red/blue configurations may be physically linked to the formation of LRDs, in analogy with the "synchronized pair" scenario originally proposed for direct-collapse black hole formation. In this picture, ultraviolet radiation from the companions, which typically have modest stellar masses ($M_\ast \sim 10^{8-9}M_\odot$), suppresses molecular hydrogen cooling in nearby gas, allowing nearly isothermal collapse and the formation of extremely compact objects, such as massive black holes or quasi-stars. Using component-resolved photometry and SED modeling, we infer Lyman-Werner radiation fields of $J_{21,LW} \sim 10^{2.5}$-$10^{5}$ at the locations of the red components, comparable to those required in direct-collapse models, suggesting that the necessary photodissociation conditions are realized in many LRD systems. This framework provides a simple and self-consistent explanation for the extreme compactness and distinctive spectral properties of LRDs, and links long-standing theoretical models for early compact object formation directly to a population now observed with JWST in the early universe.

Short digest

Across 83 spectroscopic LRDs with JWST imaging, the authors decompose red/blue components and show that ~43% host UV-bright companions at 0.5–5 kpc (median 1.0 kpc), rising to ≳85% for the most luminous systems; in the lensed A383-LRD and newly found A68-LRD, the separations tighten to d≈0.3 kpc. These companions are modest galaxies (M* ~10^8–10^9 Msun) whose UV output yields intense local Lyman–Werner fields at the red components, J21,LW ~10^{2.5}–10^{5}. The inferred fluxes meet or exceed direct-collapse thresholds that suppress H2 cooling, naturally explaining the extreme compactness and distinctive SEDs of LRDs. This links the ubiquitous red/blue pairs to a synchronized-pair pathway for heavy-seed formation in the early universe.

Key figures to inspect

• Figure 1: Scan the RGB cutouts for the recurrent red/blue morphology and note which systems are strongly lensed (magnifications labeled), setting expectations for sub-kpc separations that may be unresolved in unlensed fields.
• Figure 2: Inspect GALFIT residuals after subtracting the red component; the checkmark vs question-mark flags show where faint UV companions are securely isolated, revealing how often blue light survives model subtraction near the red core.
• Figure 3: For A68-LRD1, compare the NIRCam cutouts to the SED decomposition; verify the Cliff-like red SED and the flat blue SED, and use the stacked intrinsic SEDs (right panel) to see how typical the red/blue split is across the companion-hosting subsample.
• Figure 4: Read the companion LW magnitude vs effective separation plane; identify which systems lie above the J21,crit line and how lensing-driven small separations push A383-LRD/A68-LRD into the high-J21 regime supporting direct collapse.