21cmFAST: A Fast, Semi-Numerical Simulation of the High-Redshift 21-cm Signal (1003.3878v1)

Abstract: We introduce a powerful semi-numeric modeling tool, 21cmFAST, designed to efficiently simulate the cosmological 21-cm signal. Our code generates 3D realizations of evolved density, ionization, peculiar velocity, and spin temperature fields, which it then combines to compute the 21-cm brightness temperature. Although the physical processes are treated with approximate methods, we compare our results to a state-of-the-art large-scale hydrodynamic simulation, and find good agreement on scales pertinent to the upcoming observations (>~ 1 Mpc). The power spectra from 21cmFAST agree with those generated from the numerical simulation to within 10s of percent, down to the Nyquist frequency. We show results from a 1 Gpc simulation which tracks the cosmic 21-cm signal down from z=250, highlighting the various interesting epochs. Depending on the desired resolution, 21cmFAST can compute a redshift realization on a single processor in just a few minutes. Our code is fast, efficient, customizable and publicly available, making it a useful tool for 21-cm parameter studies.

• The paper introduces 21cmFAST, a simulation that efficiently models the high-redshift 21-cm signal using semi-numerical techniques.
• It benchmarks the tool against hydrodynamic simulations, achieving power spectrum accuracy within tens of percent variance.
• The tool enables rapid exploration of reionization parameter spaces and supports upcoming 21-cm interferometer observations.

Overview of 21cmFAST: A Fast, Semi-Numerical Simulation of the High-Redshift 21-cm Signal

The paper by Mesinger et al. introduces 21cmFAST, a semi-numerical tool for simulating the cosmological 21-cm signal. This tool efficiently produces 3D simulations of various cosmological fields such as density, ionization, peculiar velocities, and spin temperatures, which collectively determine the 21-cm brightness temperature of high-redshift hydrogen. The authors benchmark 21cmFAST against current large-scale hydrodynamic simulations, demonstrating impressive agreement in relevant scales for future observatory data reaching up to 1 Mpc.

Simulation Efficiency and Accuracy

21cmFAST focuses on computational speed and scalability without significant sacrifices in accuracy. Realizations depicting redshift fields can be generated in mere minutes using a single processor, significantly outperforming hydrodynamic simulations, which necessitate extensive supercomputing resources and time. Despite using approximate approaches, power spectra from 21cmFAST match hydrodynamic simulations within tens of percent variance, affirming its practical use for researchers exploring 21-cm experimental data across vast parameter spaces.

Cosmological Fields and Physics

The simulation encompasses prominent cosmological physics such as the evolution of density via perturbation theory, ionization fields estimated through excursion set formalism, and peculiar velocity fields affecting brightness temperature through redshift space distortions. These components undergo validation against tailored hydrodynamic simulations, exhibiting a high degree of congruence across various statistical measures, including probability density functions (PDFs) and power spectra.

Of particular note is the investigation of non-linear effects due to peculiar velocities. 21cmFAST identifies a notable enhancement of the 21-cm power spectrum beyond expected linear geometric factors, underscoring the complexity of redshift space distortions in cosmological 21-cm observations. This study contributes to understanding how such effects are mitigated or accentuated during different phases of reionization.

Pre-Reionization Regime and Spin Temperature

In tracking the evolution from $z=250$, 21cmFAST also simulates the pre-reionization era, incorporating astrophysics such as X-ray heating and the Wouthuysen-Field (WF) effect. The authors detail the intricate stages before reionization, characterized by phases including collisional coupling and decoupling, WF coupling, and X-ray heating. This broad spectrum analysis provides researchers with insights into the high-redshift universe's temperature field, especially valuable given the complexity and unknowns surrounding early cosmic epochs.

Implications and Future Prospects

21cmFAST proves to be a robust alternative to computationally expensive numerical simulations, offering rapid simulations essential for parameter studies. Its applicability to upcoming observations by major 21-cm interferometers, such as LOFAR and the Square Kilometer Array (SKA), is significant. With upcoming data that are expected to fill present observational gaps in high-redshift studies, 21cmFAST stands as a pivotal tool in understanding early cosmic phenomena, facilitating exploration of the parameter space related to galaxy formation and evolution during reionization.

Finally, 21cmFAST's public availability encourages widespread adoption and adaptation within the astrophysical community, promoting collaborative advances and detailed cross-analysis of 21-cm related datasets. As observational capacities improve, 21cmFAST's contribution to the cosmological landscape will only grow, serving as a linchpin in bridging theoretical predictions with observed realities.