Centrally Concentrated H i Distribution Enhances Star Formation in Galaxies

We use a sample of 13,511 nearby galaxies from the ALFALFA and Sloan Digital Sky Survey spectroscopic surveys to study the relation between the spatial distribution of H i 21 cm emission and star formation rate (SFR). We introduce a new nonparametric quantity K, measured from the curve of growth of the line, to describe the shape of the integrated H i profile. The value of K increases from double-horned to single-peaked profiles, depending on projection effects and the spatial and velocity distribution of the gas. Using carefully chosen samples to control for the competing factors that influence the integrated line profile, we argue that useful inferences can be made on the spatial distribution of the gas. We find that galaxies with a high value of K tend to have more centrally concentrated H i distribution within the optical disk of the galaxy at fixed conditions, and that larger values of K are associated with higher levels of total and central SFR. The results suggest that the global concentration of H i plays an important role in facilitating the conversion of neutral atomic hydrogen to molecular hydrogen gas, which, in turn, affects the star formation activity throughout the optical disk. Our sample is biased against quiescent galaxies, and thus the conclusions may not hold for galaxies with low SFR or low H i content.