An H I Study of a Large Sample of Ultradiffuse Galaxies

By crossmatching the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) catalog with the FASHI and ALFALFA H i surveys, we construct an H i -detected sample of 112 ultradiffuse galaxies (UDGs) and 48 low-surface-brightness (LSB) galaxies, providing H i -based redshifts for 76 galaxies for the first time. Using DESI DR1 redshifts, we assemble an H i -non-detected sample of 168 galaxies for a stacking analysis, with detections in two stellar-mass bins below 10 8.5 M ⊙ . Combining optical, UV, and H i data, we investigate stellar masses, star formation rates, gas fractions, and kinematics. We present a systematic analysis of the H i mass–optical size relation for UDGs and LSBs, showing both populations follow a mass–optical size scaling similar to normal galaxies, suggesting an average H i surface density, under the assumption that the optical size traces the extent of the H i distribution. Their stellar-mass–size relation indicates nearly constant stellar surface densities for galaxies with the central surface brightness μ 0 , g ≳ 24 mag 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 H i kinematics. Both populations exhibit low star formation efficiencies and long gas depletion times, supporting multiple formation pathways for UDGs.