xGASS: passive disks do not host unexpectedly large reservoirs of cold
  atomic hydrogen
  We use the extended GALEX Arecibo SDSS Survey (xGASS) to quantify the
relationship between atomic hydrogen (HI) reservoir and current star formation
rate (SFR) for central disk galaxies. This is primarily motivated by recent
claims for the existence, in this sample, of a large population of passive
disks harbouring HI reservoirs as large as those observed in main sequence
galaxies. Across the stellar mass range 10$^{9}<$M$_{*}$/M$_{\odot}<$10$^{11}$,
we practically find no passive ($\gtrsim$2$\sigma$ below the star-forming main
sequence) disk galaxies with HI reservoirs comparable to those typical of
star-forming systems. Even including HI non detections at their upper limits,
passive disks typically have $\geq$0.5 dex less HI 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 catalog, which do
not provide a fair representation of the global SFR of HI-rich galaxies with
extended star-forming disks. Our findings confirm that the bulk of the passive
disk population in the local Universe is HI-poor. These also imply that the
reduction of star formation, even in central disk galaxies, has to be
accompanied by a reduction in their HI reservoir.
