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

Abstract Neutral atomic hydrogen (Hi) reservoirs typically extend far beyond the inner star-forming regions of galaxies, and global Hi measurements, which mix these distinct environments, limit our understanding of the gas–star formation cycle. In particular, global Hi depletion times combine gas and star formation from different physical scales, contributing to long measured timescales (5–9 Gyr) and large scatter compared to molecular gas. Using 841 gas-rich galaxies from the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) pilot observations, we investigate how Hi depletion time and its scaling relations change when Hi 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 Hi remains inactive for star formation. Hi depletion times anti-correlate strongly with stellar surface density, and this trend becomes even tighter within the stellar disc. The Kennicutt–Schmidt relation further reveals an almost constant Hi depletion time at fixed stellar surface density, similar to the behaviour seen for molecular gas, suggesting that Hi and star formation are regulated by conditions that enable Hi–to–H2 conversion, traced by stellar surface density. Beyond the stellar disc, Hi 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 Hi to serve as a fuel for star formation.