2020MNRAS.494L..42C

We use the extended GALEX Arecibo SDSS Survey (xGASS) to quantify the relationship between atomic hydrogen (H I) reservoir and current star formation rate (SFR) for central disc galaxies. This is primarily motivated by recent claims for the existence, in this sample, of a large population of passive discs harbouring H I reservoirs as large as those observed in main-sequence galaxies. Across the stellar mass range 10⁹ < M_*/M_☉ < 10¹¹, we practically find no passive (>=2σ below the star forming main sequence) disc galaxies with H I reservoirs comparable to those typical of star-forming systems. Even including H I non-detections at their upper limits, passive discs typically have >=0.5 dex less H I than their active counterparts. We show that previous claims are due to the use of aperture-corrected SFR estimates from the MPA/JHU SDSS DR7 catalogue, which do not provide a fair representation of the global SFR of H I-rich galaxies with extended star-forming discs. Our findings confirm that the bulk of the passive disc population in the local Universe is H I-poor. These also imply that the reduction of star formation, even in central disc galaxies, has to be accompanied by a reduction in their H I reservoir.