2025ApJ...992...79H

Recent observations with the Five-hundred-meter Aperture Spherical Telescope have revealed abundant reservoirs of neutral hydrogen (H I) in low-redshift poststarburst galaxies (PSBs), raising the question of why star formation ceases rapidly in these systems. In this study, we present a detailed analysis of the shape of the integrated H I spectra of 67 PSBs. We find that PSBs exhibit significantly higher H I spectral concentration values (K) compared to a matched sample from xGASS, and are more comparable to those of starburst galaxies. By extending our analysis to spatially resolved H I data from THINGS and ATLAS^3D, we show that both centrally concentrated H I distributions and dynamically unsettled H I can effectively increase K, while nonaxisymmetric structures only contribute to the scatter of the K distribution. Distinguishing between central concentration and dynamically unsettled gas as the origin of high K can be achieved by measuring the spectral asymmetry (A_F), making the K-A_F plane a powerful diagnostic tool for identifying galaxies with unsettled H I using integrated spectra alone. Based on their location in the K-A_F plane, we find that most PSBs are not dominated by unsettled H I, but rather exhibit elevated central gas concentration. Both modes of gas redistribution in PSBs may eventually contribute to their quenching.