PRIYA: A New Suite of Lyman-alpha Forest Simulations for Cosmology (2306.05471v2)

Abstract: We present the PRIYA suite of cosmological simulations, based on the code and hydrodynamic model of the ASTRID simulation, and designed for cosmological analyses of the Lyman-$\alpha$ forest. Our simulation suite spans a $9$-dimensional parameter space, including $4$ cosmological parameters and $5$ astrophysical/thermal parameters. We have run $48$ low fidelity simulations with $1536^3$ particles in a $120$ Mpc/h box and $3$ high fidelity simulations with $3072^3$ particles in a $120$ Mpc/h box. All our simulations include a full physics model for galaxy formation, including supernova and AGN feedback, and thus also contain a realistic population of DLAs. We advance on earlier simulations suites by larger particle loads, by incorporating new physical models for patchy hydrogen and helium reionization, and by self-consistently incorporating a model for AGN feedback. We show that patchy helium reionization imprints an excess in the 1D flux power spectrum on large scales, which may allow future measurements of helium reionization bubble sizes. Simulation parameters are chosen based on a Latin hypercube design and a Gaussian process is used to interpolate to arbitrary parameter combinations. We build a multi-fidelity emulator for the 1D flux power spectrum and the mean IGM temperature. We show that our final interpolation error is $< 1\%$ and that our simulations produce a flux power spectrum converged at the percent level for $z=5.4$ - $2.2$. Our simulation suite will be used to interpret Lyman-$\alpha$ forest 1D flux power spectra from SDSS and future DESI data releases.

• The paper introduces the PRIYA suite, a simulation framework that combines multi-fidelity models to probe Lyman-alpha forest dynamics.
• It employs a nine-dimensional parameter space including key cosmological and astrophysical factors, with explicit modeling of patchy reionization.
• Results show robust convergence in flux power spectrum predictions, providing a precise tool for analyzing current and future cosmological surveys.

Overview of PRIYA: A New Suite of Lyman-$\alpha$ Forest Simulations for Cosmology

The paper presents the PRIYA suite of cosmological simulations, specifically designed to analyze the Lyman-$\alpha$ forest—a critical probe of the small-scale structure in the universe—and explores the impact of various cosmological and astrophysical parameters. The PRIYA suite builds upon the ASTRID project's computational framework, extending its capabilities through an intricate sampling of parameter space, the incorporation of novel models for patchy hydrogen and helium reionization, and the development of a sophisticated multi-fidelity emulator.

Simulation Parameters and Design

The PRIYA simulations encompass a nine-dimensional parameter space, integrating both cosmological and astrophysical quantities. These include:

• Four cosmological parameters: the scalar spectral index $n_P$, the amplitude $A_P$ of perturbations, the Hubble parameter $h$, and the growth rate $\Omega_M h^2$.
• Five astrophysical parameters related to reionization processes and feedback mechanisms.

The simulation suite consists of 48 low fidelity and three high fidelity simulations, executed within a 120 Mpc/h box with particle resolutions of $1536^3$ and $3072^3$. The simulations account for galaxy formation physics, including supernova and AGN feedback, and are executed using a Latin hypercube design supplemented with a Gaussian process emulator to facilitate interpolation across parameter space.

Methodological Advances

A significant advancement in these simulations is the explicit modeling of patchy hydrogen and helium reionization, which allows for a more detailed examination of thermal histories and their signatures in the Lyman-$\alpha$ forest flux power spectrum. The simulations introduce a spatially varying ultra-violet background, applying semi-analytic models that simulate the progression of reionization from quasars and their bubble structures.

This approach represents a notable improvement in capturing the IGM temperature evolution, which is validated against observational data, bridging the gap between physical plausibility and empirical thermal histories.

Emulator Development

A cornerstone of the paper is the development of a multi-fidelity emulator, which fuses the resolution of the high fidelity simulations with the extensive coverage of the low fidelity ones. This emulator achieves sub-percent interpolation errors thanks to the improved design of the low fidelity simulations and efficiently predicts the 1D flux power spectrum and IGM temperatures. The authors leverage Bayesian optimization to ensure the fidelity and distribution of simulation parameter points.

Key Results and Implications

The simulations show robust convergence in flux power spectrum predictions relative to box size and resolution—essential for reliable cosmological inference. The prediction errors from the emulator are considerably below measurement uncertainties, marking the accuracy needed for application to current and future datasets, including SDSS and DESI.

The feedback from AGN processes, often vital in simulating galaxy formation, is shown to have a negligible effect on the Lyman-$\alpha$ forest flux power spectrum over the scales examined, suggesting the dominance of the underlying cosmological parameters at these redshifts and scales.

Implications and Future Directions

The PRIYA simulation suite forms a foundational tool for interpreting high-precision Lyman-$\alpha$ forest datasets and offers a pathway for detailed exploration of reionization models via the detected large-scale biases imprinted by inhomogeneous reionization—both hydrogen and helium. The multi-fidelity approach maximizes computational efficiency and enables a wide-ranging exploration of parameter space, allowing for potential extensions to other cosmological probes and joint analyses with other baryonic effects.

In future applications, these simulations are poised to refine constraints on small-scale power in the universe, provide insights into IGM thermal histories, and further our understanding of the interplay between cosmic structures at high redshift, acting as a benchmark for the assimilation and synthesis of diverse astrophysical and cosmological inquiries.