The Emergence of Little Red Dots from Binary Massive Black Holes

TL;DR

This paper proposes a model where Little Red Dots are binary black hole systems with specific disk structures, explaining their spectra, abundance at high redshift, and evolution, linking them to gravitational wave sources.

Contribution

It introduces a novel binary black hole disk model that explains LRD spectral features and their evolution, connecting them to GW-detectable mergers.

Findings

LRDs' spectra are explained by binary black hole systems with circum-binary and mini-disks.

The model accounts for the high redshift abundance and spectral shape of LRDs.

Predicted spectral evolution links early binaries to the broader AGN population and late binaries to GW sources.

Abstract

Little red dots (LRDs) are a newly identified class of broad-line active galactic nuclei (AGN) with a distinctive v-shape spectrum characterized by red optical and blue UV continuum emission. Their high abundance at redshifts of $z\sim6-8$ and decline at lower redshifts suggest a transient origin. We propose that the spectral shape of LRDs originates from compact binary black hole systems, where each black hole is surrounded by a mini-disk and embedded in a larger circum-binary disk. With a binary separation of $\lesssim 10^3$ Schwarzschild radii, the Wien tail of a $T\simeq 5000~{\rm K}$ blackbody spectrum at the inner edge of the circum-binary disk produces the red optical emission, while the mini-disks power the UV continuum. Binary torques carve out a gap between the circum-binary disk and mini-disks, setting the turnover wavelength of the v-shaped spectrum around the Balmer limit. This scenario naturally reproduces LRD spectra requiring only modest dust attenuation ($A_V\lesssim 1$ mag), resolving overestimated luminosities for LRDs in previous studies and alleviating a tension with the so-called Soltan argument. This model predicts a distinct spectral evolution as the binary orbit decays through binary-disk interactions and gravitational waves (GWs), linking early-stage "proto-LRD" binaries to the broader AGN population and late-stage "LRD-descendants" to coalescing binaries detectable in GW experiments.