An HI study of a large sample of ultra-diffuse galaxies
By cross-matching the SMUDGes catalog with the FASHI and ALFALFA HI surveys, we construct an HI-detected sample of 112 ultra-diffuse galaxies (UDGs) and 48 low-surface-brightness (LSB) galaxies, providing HI-based redshifts for 76 galaxies for the first time. Using DESI DR1 redshifts, we assemble an HI-non-detected sample of 168 galaxies for stacking analysis, with detections in two stellar mass bins below 10^{8.5} M_sun. Combining optical, UV, and HI data, we investigate stellar masses, star formation rates, gas fractions, and kinematics. We present a systematic analysis of the HI mass--optical size relation for UDGs and LSBs, showing both populations follow a mass--optical size scaling similar to normal galaxies, suggesting an average HI surface density, under the assumption that the optical size traces the extent of the HI distribution. Their stellar mass--size relation indicates nearly constant stellar surface densities for galaxies with central surface brightness mu_{0,g}>~24mag arcsec^{-2}, independent of UDG or LSB classification. Many UDGs and LSB galaxies are systematically offset from the baryonic Tully--Fisher relation toward higher baryonic masses at a given velocity, consistent with trends found in UDGs with resolved HI kinematics. Both populations exhibit low star formation efficiencies and long gas depletion times, supporting multiple formation pathways for UDGs.