The H i angular momentum–mass relation

Abstract

ABSTRACT We study the relationship between the H i specific angular momentum ($\rm j_{g}$) and the H i mass ($\rm M_{g}$) 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 $\rm j_{g}$ ∝ $\rm M_{g}$α over the entire mass range (107–1010.5 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 ∝ M2/3 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.