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

Abstract

The Little Red Dots (LRDs) are a population of compact, red sources, detected abundantly at $4 \lesssim z \lesssim 9$. While their nature is still debated, many models suggest they are powered by accreting massive black holes. Multiple studies have detected blue companions in their vicinity, motivating our analysis: 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 host a blue ($β<-1$) companion at $0.5-2$ kpc, versus $11.6\%$ of redshift-matched controls: a $\times 2.39$ excess, significant at $3.3σ$. 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. The companions of the brightest LRDs are overwhelmingly ($92\%$) blue, and significantly bluer, with a median $β\approx-2.32$, versus $β\approx-1.92$ for those of controls, rejecting a common parent distribution at $3.4σ$. The signal is thus directional in three independent ways: luminosity, separation, and companion color, all 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 for why the most luminous LRDs have nearby, young, star-forming neighbors is that the radiation from these neighbors enabled the direct collapse of the massive black holes powering the LRDs themselves.