Constraints on Reionization from Planck Low-ℓ EE Polarization and the Lyman-Series Forests
Abstract
CMB measurements tightly constrain the Thomson scattering optical depth (τes), but they still permit a broad range of degenerate reionization histories. Incorporating Lyman-series forest constraints on the end of reionization can alleviate the degeneracy and narrow the range of allowed histories. We explore this effect using forest measurements combined with full Planck likelihoods. We apply upper limits on the neutral fraction at 4.9≲ z≲6.9 derived from the dark pixel fraction, as well as a lower limit at z=5.8 from damping-wing-like features on the edges of long dark gaps in stacked forest spectra. We find that neither the choice of parametric vs. non-parametric reionization model, nor the choice among different forest data combinations, produces a significant shift in τes or the other cosmological parameters. Instead, forest constraints primarily redistribute τes in redshift. Combining dark pixel, damping-wing-like, and CMB measurements, we find z50=7.7+0.7−0.8 and a 95\% upper limit of Δz90<11.4 for the midpoint and duration of reionization, respectively. The asymmetry of the reionization history (Az) is not well constrained by these data. Stronger constraints can be obtained using Lyα forest measurements of the mean IGM photoionization rate, at the cost of requiring more detailed modeling. Using a new semi-analytic reionization model, we obtain (model-dependent) constraints of z50 = 7.4+0.2−0.3, Δz90=6.0+0.8−1.0, and Az = 2.4+0.3−0.7. We also find evidence that the ionizing emissivity of reionization sources declines after z=7−8, perhaps signaling a decline in the escape fraction and/or the onset of a feedback process. Our results motivate the development of forest likelihood products and fast forward models for its transmission statistics.
Related
- Tangled τ: Planck CMB Constraints on Flash Reionization
- Inflationary models in a minimally coupled f(R,T) gravity: Constraints from Planck, BICEP/Keck, and ACT
- Lyman-α forest 1D flux power spectrum constraints on QSO-assisted reionization models
- Resonantly Scattered CIV Emission in Local Star-forming Galaxies: Radiative Transfer Constraints on High-ionization Gas in Reionization Analogs
- Prospects for Near-Far Constraints on Reionization from Simulated LSST Observations of Milky Way Dwarf Galaxies and Stellar Streams
BibTeX
@misc{lopez2026constraints,
title = {{Constraints on Reionization from Planck Low-$\ell$ EE Polarization and the Lyman-Series Forests}},
author = {Garett Lopez and Anson D'Aloisio and Christopher Cain},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.28765},
eprint = {2610.28765},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.28765},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Constraints on Reionization from Planck Low-ℓ EE Polarization and the Lyman-Series Forests
AU - Garett Lopez
AU - Anson D'Aloisio
AU - Christopher Cain
PY - 2026
DA - 2026/10/20
PB - arXiv
DO - 10.48550/arxiv.2610.28765
UR - https://arxiv.org/abs/2610.28765
AB - CMB measurements tightly constrain the Thomson scattering optical depth (τ_es), but they still permit a broad range of degenerate reionization histories. Incorporating Lyman-series forest constraints on the end of reionization can alleviate the degeneracy and narrow the range of allowed histories. We explore this effect using forest measurements combined with full Planck likelihoods. We apply upper limits on the neutral fraction at 4.9≲ z≲6.9 derived from the dark pixel fraction, as well as a lower limit at z=5.8 from damping-wing-like features on the edges of long dark gaps in stacked forest spectra. We find that neither the choice of parametric vs. non-parametric reionization model, nor the choice among different forest data combinations, produces a significant shift in τ_es or the other cosmological parameters. Instead, forest constraints primarily redistribute τ_es in redshift. Combining dark pixel, damping-wing-like, and CMB measurements, we find z_50=7.7^+0.7_−0.8 and a 95\% upper limit of Δz_90<11.4 for the midpoint and duration of reionization, respectively. The asymmetry of the reionization history (A_z) is not well constrained by these data. Stronger constraints can be obtained using Lyα forest measurements of the mean IGM photoionization rate, at the cost of requiring more detailed modeling. Using a new semi-analytic reionization model, we obtain (model-dependent) constraints of z_50 = 7.4^+0.2_−0.3, Δz_90=6.0^+0.8_−1.0, and A_z = 2.4^+0.3_−0.7. We also find evidence that the ionizing emissivity of reionization sources declines after z=7−8, perhaps signaling a decline in the escape fraction and/or the onset of a feedback process. Our results motivate the development of forest likelihood products and fast forward models for its transmission statistics.
ER -