What Is the True H i Gas Content in Massive Quiescent Galaxies in the Local Universe?

Abstract While massive quiescent galaxies are known to be poor in atomic hydrogen (H i ), their true H i content remains poorly constrained due to the limited sensitivity and morphological biases of existing surveys. We present deep H i observations using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) for a representative sample of 78 low-redshift massive quiescent galaxies, selected by stellar mass ( M * > 10 10 M ⊙ ), color (near-UV, hereafter NUV, − r > 5), and specific star formation rate ( log sSFR < − 11 yr − 1 ). Our observations reach a remarkable sensitivity of log ( M HI / M * ) = − 2.6 for 55 targets and log ( M HI / M * ) = − 3.2 for 23 targets. We find that one-third of the sample follows the H i scaling relation derived from previous surveys, while the remaining two-thirds exhibit significantly lower H i content. The H i mass fraction shows no clear correlation with specific star formation rate, NUV − r color index, stellar surface mass density, and concentration. Our FAST sample shows remarkable similarity to the ATLAS 3D sample, which only includes early-type galaxies, both in its high fraction of H i -poor galaxies and its high satellite fraction among H i -poor galaxies. These results suggest that, while both early-type morphology and environment may contribute to the extreme H i deficiency, neither factor alone fully explains the observed gas depletion, indicating that additional physical mechanisms must be responsible for the extreme H i deficiency prevalent in massive quiescent galaxies.