Title: 21cmEMUv3: A Hybrid Diffusion-LSTM Emulator for 21cmFAST Summary Observables

Abstract

The burgeoning era of cosmic dawn (CD) and epoch of reionisation (EoR) observations is fueling a growing need for theoretical frameworks that are both rapid and resilient. To meet this challenge, machine learning (ML), specifically emulation, has become a vital tool for streamlining and improving inference processes. This study introduces 21cmEMUv3, an emulation model trained on 21cmFASTv3 simulations that account for galaxy cooling via both atomic and molecular mechanisms.

Conditioned on $\sigma_8$ alongside ten astrophysical parameters, the emulator generates seven distinct summary observables: (i) the cylindrical 21cm power spectrum (PS), which is emulated for the first time with high resolution and precision across a broad redshift interval 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) ultraviolet (UV) luminosity functions (LFs); and (vii) the Thomson scattering optical depth. The model employs a hybrid architecture: score-based diffusion techniques are used for the cylindrical 21cm PS, while long-short term memory (LSTM) networks handle the remaining six metrics. All outputs achieve sub-percent median accuracy.

We apply the emulator to reanalyze existing 21cm PS upper limits from HERA, leveraging state-of-the-art hydrodynamical simulations to establish informed priors on star formation within molecularly cooling galaxies. Our analysis indicates that the soft-band X-ray luminosity per unit star formation rate aligns with extrapolations of high-mass X-ray binaries into the low-metallicity environments anticipated in early galaxies. Specifically, we exclude values lower than $10^{39.2}$ erg s$^{-1}M^{-1}_\odot \rm{yr}$ at the 95% confidence level. Additionally, we provide detection forecasts for the cosmic 21cm PS using the Square Kilometre Array across various array configurations. The 21cmEMU package is publicly accessible.

Source: arXiv Generated at: 2026-06-02 00:00:00 UTC