21cmEMUv3: a hybrid diffusion-LSTM emulator of 21cmFAST summary observables

2026-05-29

AI Summary

Observational data from the Cosmic Dawn and Epoch of Reionization are rapidly accumulating, creating an urgent need for efficient and robust theoretical modeling tools. This work proposes a hybrid diffusion–LSTM surrogate model based on 21cmFASTv3 simulations to jointly generate seven key 21cm summary statistics across a broad redshift range (z ≈ 6–30). The approach employs a score-based diffusion model to synthesize high-fidelity, cylindrically averaged 21cm power spectra, while the remaining six statistics are modeled using LSTM networks. The framework achieves median accuracies at the sub-percent level and successfully reinterprets current HERA upper limits, constraining the minimum X-ray luminosity per star formation rate in low-metallicity environments to 10³⁹·² erg s⁻¹ M⊙⁻¹ yr. It further forecasts the detection capabilities of various SKA configurations.

Abstract

We are witnessing a surge in observations of the cosmic dawn (CD) and epoch of reionisation (EoR), driving an increasing demand for fast and robust theoretical interpretation frameworks. In response, machine learning (ML), and emulation in particular, has emerged as a powerful approach to accelerate and enhance inference pipelines. In this work, we present 21cmEMUv3, an emulator trained on 21cmFASTv3 simulations that model both atomically and molecularly cooling galaxies. 21cmEMUv3 is conditioned on $σ_8$ and ten astrophysical parameters to produce seven summary observables: (i) the cylindrical 21cm power spectrum (PS), emulated for the first time at such high resolution and accuracy across a wide redshift range of $z \sim$ 6--30; (ii) the spherically-averaged 21cm PS; (iii) the mean neutral fraction of the intergalactic medium (IGM); (iv) the mean 21cm spin temperature; (v) the global 21cm signal; (vi) the ultraviolet (UV) luminosity functions (LFs); and (vii) the Thomson scattering optical depth. Notably, the cylindrical 21cm PS is emulated via score-based diffusion, while the remaining six summaries are emulated via long-short term memory (LSTM) networks, all achieving sub-percent median accuracy. We use the emulator to reinterpret current 21cm PS upper limits from HERA, for the first time using state-of-the-art hydrodynamical simulations to inform priors on star formation inside molecularly cooling galaxies. We find that our inferred soft-band X-ray luminosity per unit star formation rate is consistent with extrapolations of high-mass X-ray binaries to the low-metallicity regimes expected in the first galaxies, excluding values below $10^{39.2}$ erg s$^{-1}M^{-1}_\odot \rm{yr}$ at $95\%$ confidence. Finally, we produce forecasts for the detection of the cosmic 21cm PS with the Square Kilometre Array for different array configurations. The 21cmEMU package is publicly available.

Problem

• cosmic dawn
• epoch of reionisation
• 21cm signal
• emulation
• astrophysical inference

Innovation

• hybrid diffusion-LSTM emulator
• 21cm power spectrum
• cosmic dawn
• epoch of reionisation
• score-based diffusion