Kavli Affiliate: Jinyi Shangguan
| First 5 Authors: Kohei Inayoshi, Jinyi Shangguan, Xian Chen, Luis C. Ho, Zoltan Haiman
| Summary:
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
$zsim6-8$ 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 $Tsimeq 5000~{rm K}$ 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
($A_Vlesssim 1$ 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.
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