FAST Drift-scan Survey for H i Intensity Mapping: Simulation on Hunting H i Filament with Pairwise Stacking

Filaments stand as pivotal structures within the cosmic web. However, direct detection of the cold gas content of the filaments remains challenging owing to its inherently low brightness temperature. With the TNG hydrodynamical simulations, we demonstrate the effectiveness of isolating faint filament H i signal from the FAST H i intensity mapping (IM) survey through pairwise stacking of galaxies, which yields an average H i filament signal amplitude of ∼0.29 μK at z ≃ 0.1. However, our simulations reveal a nonnegligible contribution from H i-rich galaxies within or near the filaments. Particularly, the faint galaxies dominantly contribute to the extra filament H i signal. Our simulation also shows that the measurement uncertainty is produced by both thermal noise and background variation caused by brightness leakage from surrounding random galaxies. Given a fixed total observation time, a wide-field H i IM survey, which includes a large number of galaxy pairs, can simultaneously reduce thermal noise to below the filament signal level and minimize background variation to a negligible level. Through the end-to-end simulation, this work demonstrates the critical role of the galaxy pairwise stacking method in future filament H i detection, outlining a road map for filament H i detection in the next-generation H i IM surveys.