2021MNRAS.507..565K

We study the relationship between the H I specific angular momentum ( j_g) and the H I mass ( M_g) for a sample of galaxies with well-measured H I rotation curves. We find that the relation is well described by an unbroken power law j_g ∝ M_g^α over the entire mass range (10⁷-10^10.5 M_☉), with α = 0.89 ± 0.05 (scatter 0.18 dex). This is in reasonable agreement with models which assume that evolutionary processes maintain H I discs in a marginally stable state. The slope we observe is also significantly different from both the j ∝ M^2/3 relation expected for dark matter haloes from tidal torquing models and the observed slope of the specific angular momentum-mass relation for the stellar component of disc galaxies. Our sample includes two H I-bearing ultra-diffuse galaxies, and we find that their angular momentum follows the same relation as other galaxies. The only discrepant galaxies in our sample are early-type galaxies with large rotating H I discs, which are found to have significantly higher angular momentum than expected from the power-law relation. The H I discs of all these early-type galaxies are misaligned or counter-rotating with respect to the stellar discs, consistent with the gas being recently accreted. We speculate that late-stage wet mergers, as well as cold flows play a dominant role in determining the kinematics of the baryonic component of galaxies as suggested by recent numerical simulations.