2025ApJ...980...25W

We present the H I surface density (Σ_H i_) radial distributions based on total-power H I images obtained by FAST in the FEASTS program, for 35 galaxies with inclinations lower than 72°. We derive the H I radius R₀₀₁, which is the radius for the 0.01 M_⊙ pc^–2 (∼10^18.1 cm^–2) isodensity level, 100 times deeper than the 1 M_⊙ pc^–2 level previously commonly used to measure R₁. The profile shapes show a large diversity at a given radius in units of kpc, group virial radius, and R₁, but they align more tightly with the radius normalized by R₀₀₁. The universal H I profile has a scatter of ∼0.2 dex and a scale length of ∼0.11R₀₀₁ in the outer region. We derive a new R₀₀₁-M_H i_ relation, which has a scatter of 0.03 dex and a similar slope of ∼​​​​​​0.5 to the previously known R₁-M_H i_ relation. Excluding strongly tidal-interacting galaxies, the ratio R₀₀₁/R₁ (anti)correlates strongly and significantly with the H I-to-stellar mass ratio and specific star formation rate, but not with the stellar mass, M_H i_, dark matter mass, or star formation rate. The strongly tidal-interacting galaxies tend to show deviations from these trends and have the most flattened profiles. These results imply that, in the absence of major tidal interactions, physical processes must cooperate so that Σ_H i_ distributes in a self-similar way in the outer region down to the 0.01 M_⊙ pc^–2 level. Moreover, they may drive gas flows in such a way that H I-richer galaxies have H I disks that not only extend further but also transport H I inward more efficiently from R₀₀₁ to R₁.