FEASTS Combined with Interferometry. II. Significantly Changed H i Surface Densities and Even More Inefficient Star Formation in Galaxy Outer Disks

Abstract

We update the H i surface density measurements for a subset of 17 THINGS galaxies by dealing with the short-spacing problem of the original Very Large Array (VLA) H i images. It is the same sample that Bigiel et al. used to study the relation between H i surface densities and star formation (SF) rate surface densities in galaxy outer disks, which are beyond the optical radius r 25. For 10 galaxies, the update is based on combining original THINGS VLA H i images with H i images taken by the single-dish FAST in the FEASTS program. The median increment of H i surface densities in outer disks is 0.15–0.4 dex at a given new H i surface density. Several galaxies change significantly in the shape of radial profiles H i surface densities, and seven galaxies are now more than 1σ below the H i size–mass relation. We update the H i star formation laws in outer disks. The median relation between the H i surface densities and SF rate surface densities based on pixel-wise measurements shifts downward by around 0.15 dex because the H i surface density values shift rightward, and the scatter increases significantly. The scatter of the relation, indicating the star-forming efficiency, exhibits a much stronger positive correlation with the stellar mass surface density than before. Thus, detecting the previously missed, diffuse H i due to the short-spacing problem of the Very Large Array observations is important in revealing the true condition and variation in SF possibly regulated by stellar feedbacks in the localized environment of outer disks.