  We present a statistical study of the properties of diffuse HI in ten nearby
galaxies, comparing the HI detected by the single-dish telescope FAST (FEASTS
program) and the interferometer VLA (THINGS program), respectively. The THINGS'
observation missed HI with a median of 23% due to the short-spacing problem of
interferometry and limited sensitivity. We extract the diffuse HI by
subtracting the dense HI, which is obtained from the THINGS data with a uniform
flux-density threshold, from the total HI detected by FAST. Among the sample,
the median diffuse-HI fraction is 34%, and more diffuse HI is found in galaxies
exhibiting more prominent tidal-interaction signatures. The diffuse HI we
detected seems to be distributed in disk-like layers within a typical thickness
of $1\,\text{kpc}$, different from the more halo-like diffuse HI detected
around NGC 4631 in a previous study. Most of the diffuse HI is cospatial with
the dense HI and has a typical column density of
$10^{17.7}$-$10^{20.1}\,\text{cm}^{-2}$. The diffuse and dense HI exhibits a
similar rotational motion, but the former lags by a median of 25% in at least
the inner disks, and its velocity dispersions are typically twice as high.
Based on a simplified estimation of circum-galactic medium properties and
assuming pressure equilibrium, the volume density of diffuse HI appears to be
constant within each individual galaxy, implying its role as a cooling
interface. Comparing with existing models, these results are consistent with a
possible link between tidal interactions, the formation of diffuse HI, and gas
accretion.
