Lyman-$α$ forest 1D flux power spectrum constraints on QSO-assisted reionization models

Vid Iršič, Avery Meiksin

First listed 2026-09-16 | Last updated 2026-09-15

Abstract

Recent JWST observations have revealed a large population of faint Quasi-Stellar Objects (QSOs) at redshifts 4 < z < 6, including a highly reddened sub-population of Little Red Dots. We exploit the sensitivity of the Lyman-$α$ forest 1D flux power spectrum to thermal fluctuations in the Intergalactic Medium (IGM) to place statistical constraints on the spectral properties of the QSOs. By post-processing simulated Lyman-$α$ forest spectra from the Sherwood-Relics suite of Lyman-$α$ forest simulations with added HeII reionization by QSOs, in conjunction with published precision measurements of the Lyman-$α$ forest 1D flux power spectrum between 4.2 < z < 5.0, we find that the addition of temperature boosts of the IGM within the HeIII regions improves agreement with the measured power spectra. While the contribution of the Little Red Dot population to HeII reionization is consistent with both a mild temperature boost of $ΔT_b=1\times10^4$ K for soft spectra QSOs and $ΔT_b=2\times10^4$ K for hard spectra QSOs, a contribution from the larger population of faint QSOs found by JWST having $M_{\rm UV} > -21.6$ is excluded at the 2$σ$ level if their spectra are sufficiently hard to boost the IGM temperature in HeIII regions by $ΔT_b=2\times10^4$ K or greater.

Short digest

Iršič and Meiksin use Sherwood-Relics Lyα-forest simulations post-processed with QSO-driven He III regions to test whether JWST’s faint AGN populations overheat the z=4.2–5.0 intergalactic medium. The added, patchy He II-reionization temperature boosts improve agreement with precision 1D flux-power measurements, whose small-scale suppression is sensitive to QSO abundance and EUV spectral hardness. Little Red Dots remain consistent with either a 10,000 K boost from soft-spectrum QSOs or a 20,000 K boost from hard-spectrum QSOs, but the much larger JWST faint-QSO population with MUV > -21.6 is excluded at 2σ when hard spectra produce boosts of at least 20,000 K. The result turns the Lyα forest into a statistical constraint on whether the newly identified faint AGN population can substantially drive early He II reionization.

Key figures to inspect

• Figure 1. This figure establishes the physical lever arm of the analysis: hotter QSO-generated He III regions suppress small-scale Lyα flux power, with the effect strengthening as the luminosity function is integrated to fainter QSOs. Its comparison of the two QSO luminosity-function models and 10,000 versus 20,000 K temperature boosts directly motivates the paper’s LRD-versus-larger-faint-QSO constraint.
• Figure 3. The posterior contours show how the Lyα forest data constrain the maximum UV magnitude of the QSO population jointly with thermal, transmission, and cosmological parameters. This is the core statistical diagnostic behind the conclusion that LRD-like number densities are allowed while a numerous hard-spectrum faint-QSO population is disfavored.
• Figure 4. This synthesis figure translates the power-spectrum inference into an ionizing-emissivity history and compares it with bright-end and faint-end QSO luminosity-function expectations, as well as an optical-depth-distribution-motivated QSO-assisted model. It clarifies the population-level implications of the Lyα constraint beyond the fitted temperature parameters.