FEASTS Combined with Interferometry. I. Overall Properties of Diffuse H I and Implications for Gas Accretion in Nearby Galaxies

We present a statistical study of the properties of diffuse H I in 10 nearby galaxies, comparing the H I detected by the single-dish telescope FAST (FEASTS program) and the interferometer Very Large Array (THINGS program), respectively. The THINGS observation missed H I with a median of 23% due to the short-spacing problem of interferometry and limited sensitivity. We extract the diffuse H I by subtracting the dense H I, which is obtained from the THINGS data with a uniform flux-density threshold, from the total H I detected by FAST. Among the sample, the median diffuse-H I fraction is 34%, and more diffuse H I is found in galaxies exhibiting more prominent tidal-interaction signatures. The diffuse H I we detected seems to be distributed in disk-like layers within a typical thickness of 1 kpc, different from the more halo-like diffuse H I detected around NGC 4631 in a previous study. Most of the diffuse H I is cospatial with the dense H I and has a typical column density of 1017.7-1020.1 cm-2. The diffuse and dense H I exhibit 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 circumgalactic medium properties and assuming pressure equilibrium, the volume density of diffuse H I 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 H I, and gas accretion.