An AI Superresolution Field Emulator for Cosmological Hydrodynamics: The Ly <i>α</i> Forest

Abstract We extend our superresolution and emulation framework for cosmological dark matter simulations to include hydrodynamics. We present a two-stage deep learning model to emulate high-resolution baryonic fields from low-resolution (LR-HydroSim) simulations at redshift z = 3. The method takes as inputs an LR-HydroSim and the high-resolution initial conditions (HR-HydroICs). First, the model stochastically generates high-resolution baryonic fields from the LR-HydroSim. Second, a deterministic emulator refines these fields using HR-HydroICs to reconstruct small-scale structures, including displacement, velocity, internal energy, and gas/star classification. Trained on paired low- and high-resolution simulations produced with MP-Gadget , the model captures small-scale structures of the intergalactic medium and observables down to the 100 kpc pressure smoothing scale relevant to the Ly α forest. The model achieves subpercent error for overdensity, temperature, velocity, and optical depth fields, a mean relative error of 1.07% in the large-scale flux power spectrum ( k &lt; 3 × 10 −2 s km −1 ), and less than 10% error in the flux probability distribution function. Notably, the two-stage model reduces the computational time by a factor of ∼260 compared to full smoothed particle hydrodynamics at the same resolution. This work demonstrates the potential of this framework as a powerful and efficient tool for generating high-resolution fields offering fast and accurate alternatives to traditional cosmological hydrodynamic simulations and enabling large-volume mock datasets for next-generation cosmological surveys.