Researchers from Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences and and collaborators have shown that the apparent “early galaxy crisis” triggered by recent James Webb Space Telescope (JWST) observations may not require a breakdown of the standard cosmological model. Instead, it may arise from a structural degeneracy in how reionization observations are interpreted.

The study, led by Zihan Wang and Huanyuan Shan, has been published in The Astrophysical Journal Letters.

JWST has discovered surprisingly bright galaxies at redshifts (z>10), corresponding to an epoch when the Universe was only a few hundred million years old. These galaxies appear to have formed too early and too efficiently, leading to claims that they may challenge the standard ΛCDM cosmological model.

The new study shows that this tension is closely related to a long-standing puzzle in reionization studies: estimates of the ionizing escape fractionhave differed by factors of several for more than a decade. The team found that global reionization observations mainly constrain the product of the escape fraction and the star-formation efficiency, rather than either quantity separately. As a result, apparently inconsistent escape-fraction estimates can be understood as different projections of the same underlying degeneracy.

By using the shape of the JWST ultraviolet luminosity function to independently constrain the star-formation efficiency, the researchers were able to break this degeneracy. Their analysis shows that the high-efficiency region previously associated with the JWST early-galaxy crisis is significantly excluded, even when stochastic bursty star formation is taken into account.

The team further reconstructed the evolution of the ionizing escape fraction over z=7–12. They found that a moderate escape fraction of about 10%–16% can naturally connect low-redshift direct measurements with high-redshift reionization constraints, providing a self-consistent picture of how early galaxies released ionizing photons and drove the Universe from neutral to ionized.

“JWST observations are rapidly changing our understanding of the cosmic dawn,” said Zihan Wang, first author of the study. “Our results suggest that the so-called early galaxy crisis may not originate from the standard cosmological model itself, but from a degeneracy among the physical parameters inferred from reionization observations. Clarifying this relationship helps place JWST’s bright early galaxies within a more consistent reionization framework.”

“This study reveals a structural limitation in early-Universe inference and opens a path for future work,” said Huanyuan Shan, corresponding author of the study. “By combining JWST spectroscopy, 21 cm reionization experiments, cosmic microwave background measurements, and large-scale galaxy surveys, we hope to trace how the first galaxies produced and released ionizing photons, and how this process influenced later galaxy formation and cosmic structure. Reionization may become a key window connecting the first galaxies, structure formation, and fundamental cosmological parameters.”

The study suggests that some apparent “crises” in the early Universe may arise from the inference framework itself rather than from a failure of the standard cosmological model. By turning a parameter degeneracy into a quantitative tool, the work provides a new way to use JWST and future 21 cm observations to probe the physics of the first galaxies.

Figure 1. Reionization degeneracy between the ionizing escape fraction and the star-formation efficiency of early galaxies. The apparently discrepant escape-fraction estimates from previous studies can be understood as different projections of the same physical constraint under different parameter assumptions.

Figure 2. JWST ultraviolet luminosity functions break the reionization-parameter degeneracy. The red shaded region marks the high star-formation-efficiency regime previously associated with the early galaxy “crisis,” while the joint observational constraints significantly exclude this region, indicating that JWST’s bright early galaxies do not necessarily require anomalously high star-formation efficiencies.

This work was supported by the National Natural Science Foundation of China.

Paper information:
Wang, Z. & Shan, H. “The JWST Early Galaxy Crisis Resolved by a Reionization Degeneracy,” The Astrophysical Journal Letters, DOI: 10.3847/2041-8213/ae88f1

Scientific contact:
Huanyuan Shan, hyshan@shao.ac.cn