More Numerous and Bluer: Companions of the Brightest Little Red Dots Differ from Control Galaxies at $>3\sigma$, Hinting at Synchronized Black Hole Seed Formation

Abstract

The Little Red Dots (LRDs) are compact, red sources at $4\lesssim z\lesssim 9$, likely powered by accreting massive black holes. Multiple studies have detected blue companions in their vicinity: we compare $253$ LRDs, uniformly selected across several JWST deep fields, against $>7000$ matched controls in the same mosaics. We find that $27.7\%$ of the brightest LRD quartile (in F444W) hosts a blue ($\beta <-1$) companion at $0.5-2$ kpc, versus $11.6\%$ of redshift-matched controls: a $\times 2.39$ excess, significant at $3.3\sigma$. Against controls additionally matched in brightness and compactness, the excess ratio is similar ($\times 2.17$), although constrained only at $1.4\sigma$ by the small size of that control pool. This phenomenon is limited to the brightest LRDs, as the full LRD sample pairs at a rate indistinguishable from controls. Moreover, the excess is confined to the close-pair window ($<2$ kpc), outside which LRD environments are ordinary. Their companions are overwhelmingly ($92\%$) blue and significantly bluer than those of controls ($\beta \approx -2.32$ versus $-1.92$), rejecting a common parent distribution at $3.4\sigma$. The signal is thus directional in brightness, separation, and companion color, as expected for synchronized-pair seed formation. Therefore, we conclude that the association between the brightest LRDs and their close blue companions is physical, i.e., the two phenomena are causally connected: either one drives the other, or both emerge from the same synchronized formation event. A natural explanation is that the radiation from these young, star-forming neighbors enabled the direct collapse of the massive black holes powering the LRDs themselves.

Accepted for publication in The Astrophysical Journal Letters. 11 pages, 4 figures