HI angular momentum mass relation

Abstract

We study the relationship between the H{\sc i} specific angular momentum (j${}_{\mathrm{g}}$) and the H{\sc i} mass (M${}_{\mathrm{g}}$) for a sample of galaxies with well measured H{\sc i} rotation curves. We find that the relation is well described by an unbroken power law \jg $\propto$ \mg${}^{\alpha }$ over the entire mass range (10${}^7$-10${}^{10.5}$ M${}_{\odot }$), with $\alpha =0.89\pm 0.05$ (scatter 0.18 dex). This is in reasonable agreement with models which assume that evolutionary processes maintain H{\sc i} disks in a marginally stable state. The slope we observe is also significantly different from both the $j\propto M^{2/3}$ 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 disk galaxies. Our sample includes two H{\sc 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{\sc i} disks which are found to have significantly higher angular momentum than expected from the power law relation. The H{\sc i} disks of all these early-type galaxies are misaligned or counter-rotating with respect to the stellar disks, 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.

Accepted for publication in MNRAS.15 pages, 22 figures, 6 tables

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