FAST drift scan survey for HI intensity mapping: simulation on hunting HI filament with pairwise stacking

TL;DR

This paper demonstrates that pairwise stacking in FAST HI intensity mapping surveys can effectively detect faint HI filaments in the cosmic web, overcoming challenges posed by low brightness and background noise.

Contribution

It introduces a simulation-based validation of the pairwise stacking method for detecting HI filaments, highlighting its potential for future large-scale HI surveys.

Findings

Average HI filament signal of ~0.29 μK at z≈0.1

Faint galaxies mainly contribute to the HI signal

Wide-field surveys can reduce noise and background effects

Abstract

Filaments stand as pivotal structures within the cosmic web. However, direct detection of the cold gas content of the filaments remains challenging due to its inherent low brightness temperature. With the TNG hydrodynamical simulations, we demonstrate the effectiveness of isolating faint filament HI signal from the FAST HI intensity mapping (IM) survey through pairwise stacking of galaxies, which yields an average HI filament signal amplitude of $\sim 0.29\ {\mu{\rm K}}$ at $z\simeq 0.1$. However, our simulations reveal a non-negligible contribution from HI-rich galaxies within or near the filaments. Particularly, the faint galaxies dominantly contribute to the extra filament HI 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 HI 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 HI detection, outlining a road map for filament HI detection in the next-generation HI IM surveys.