Super-Eddington Little Blue Dots Limited to Under 10 Percent to Match Observed Helium Reionization Timeline
The preprint shows that abundant super-Eddington Little Blue Dots would drive helium reionization earlier than observed unless their escape fractions or numbers are sharply reduced. This imposes a quantitative limit on the role of faint AGN in early cosmic reionization. The result hinges on EUV spectral assumptions and the dominance of LBDs over bright quasars.
The analysis models the extreme-UV output from super-Eddington accretion disks in faint AGN identified by JWST as Little Blue Dots. Using the observed number density and assumed escape fractions, the authors calculate the cumulative helium-ionizing photon budget and compare it against the z approximately 3 endpoint of helium reionization inferred from quasar spectra. They find that a majority super-Eddington population overproduces photons and finishes reionization too early unless escape fractions drop below 1.5 percent. The work synthesizes JWST LBD demographics with helium reionization constraints from the IGM and notes that the result tightens further if luminous quasars contribute appreciably. It highlights sensitivity to the precise EUV spectral shape of super-Eddington flows, which remains poorly calibrated. This tension directly challenges models that invoke widespread super-Eddington accretion to unify LBDs and Little Red Dots. Future constraints will come from deeper JWST spectroscopy measuring both escape fractions and EUV slopes in larger LBD samples, plus improved helium reionization maps from ELT and SKA. If the super-Eddington fraction remains low, alternative explanations for LBD properties gain support.
Cain et al.: Within 18 months, JWST Cycle 3 spectra of 200+ LBDs will show average escape fractions below 5 percent if the super-Eddington fraction exceeds 10 percent.
Sources (2)
- [1]Primary Source(https://arxiv.org/abs/2608.28745)
- [2]Supporting Source(https://arxiv.org/abs/2302.07878)