WALLABY pilot survey: H i depletion times within the stellar discs of nearby galaxies

Abstract

Neutral atomic hydrogen (H i) reservoirs typically extend far beyond the inner star-forming regions of galaxies, and global H i measurements, which mix these distinct environments, limit our understanding of the gas–star formation cycle. In particular, global H i depletion times combine gas and star formation from different physical scales, contributing to long measured time-scales (5–9 Gyr) and large scatter compared to molecular gas. Using 841 gas-rich galaxies from the WALLABY (Widefield ASKAP L-band Legacy All-sky Blind surveY) pilot observations, we investigate how H i depletion time and its scaling relations change when H i and star formation are both confined to the stellar disc (R25, the isophotal radius at 25 mag arcsec−2 in i band). We find that depletion times within this region are on average 1.4 Gyr shorter than global values, though some remain very long, indicating that a substantial fraction of H i remains inactive for star formation. H i depletion times anticorrelate strongly with stellarsurface density, and thistrend becomes even tighter within the stellar disc. The KS relation furtherreveals an almost constant H i depletion time at fixed stellarsurface density,similar to the behaviour seen for molecular gas, suggesting that H i and star formation are regulated by conditions that enable H i-to-H2 conversion, traced by stellar surface density. Beyond the stellar disc, H i depletion times are on average almost 10 Gyr longer than within R25, confirming extremely inefficient star formation in low-density outer regions. These results highlight the critical role of spatial location and local conditions for H i to serve as a fuel for star formation.