WALLABY pilot survey: Spatially resolved gas scaling relations within the stellar discs of nearby galaxies

The scatter in global atomic hydrogen (HI) scaling relations is partly attributed to differences in how HI and stellar properties are measured, with HI reservoirs typically extending beyond the inner regions of galaxies where star formation occurs. Using pilot observations from the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY), we present the first measurements of HI mass enclosed within the stellar-dominated regions of galaxies for a statistical sample of 995 local gas-rich systems, investigating the factors driving its variation. We examine how global HI scaling relations change when measurements are restricted to R-25 and R-24 - the isophotal radii at 25 and 24 mag arcsec(-2) in the i-band - and explore how the fraction of HI mass andHI surface density within these radii correlate with other galaxy properties. On average, 68% of the total HI mass is enclosed within R-25 and 54% within R-24, though significant variation exists between galaxies, ranging from similar to 20% to 100%. The fraction of HI mass within R-25 shows a mild correlation with stellar properties, with galaxies of Higher stellar mass, greater stellar surface density, or redder colours enclosing a larger fraction of their HI reservoirs. These correlations do not significantly strengthen when considering R-24. Conversely, global HI surface densities show no significant correlation with stellar mass or stellar surface density, but trends start emerging when these are measured within the inner regions of galaxies. The strongest correlation is observed with optical colour, with bluer galaxies having Higher average HI surface densities within R-25 . This trend of the average HI surface density with optical colour strengthens when we restrict from R-25 to R-24, suggesting a closer connection between inner HI reservoirs and star formation. This study underscores the value of (at least marginally) resolved HI surveys of statistical samples for advancing our understanding of the gas-star formation cycle in galaxies.