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.

中文翻译：

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H i 角动量-质量关系

我们研究了具有良好测量的 H i 旋转曲线的星系样本的 H i 比角动量 ($\rm j_{g}$) 和 H i 质量 ($\rm M_{g}$) 之间的关系。我们发现在整个质量范围 (107–1010.5 M⊙) 上，该关系可以通过完整的幂律 $\rm j_{g}$ ∝ $\rm M_{g}$α 很好地描述，其中 α = 0.89 ± 0.05 （分散 0.18 dex）。这与假设进化过程将 H i 圆盘保持在边缘稳定状态的模型合理地一致。我们观察到的斜率也与来自潮汐扭转模型的暗物质晕的预期 j ∝ M2/3 关系和观察到的圆盘星系恒星分量的特定角动量-质量关系的斜率有显着差异。我们的样本包括两个含 H i 的超扩散星系，我们发现它们的角动量遵循与其他星系相同的关系。我们样本中唯一不一致的星系是具有大型旋转 H i 盘的早期型星系，它们的角动量明显高于幂律关系的预期。所有这些早期型星系的 H i 圆盘相对于恒星圆盘都未对齐或反向旋转，这与最近吸积的气体一致。我们推测后期的湿合并以及冷流在确定星系重子分量的运动学方面起着主导作用，正如最近的数值模拟所表明的那样。发现其角动量明显高于幂律关系的预期。所有这些早期型星系的 H i 圆盘相对于恒星圆盘都未对齐或反向旋转，这与最近吸积的气体一致。我们推测后期的湿合并以及冷流在确定星系重子分量的运动学方面起着主导作用，正如最近的数值模拟所表明的那样。发现其角动量明显高于幂律关系的预期。所有这些早期型星系的 H i 圆盘相对于恒星圆盘都未对齐或反向旋转，这与最近吸积的气体一致。我们推测后期的湿合并以及冷流在确定星系重子分量的运动学方面起着主导作用，正如最近的数值模拟所表明的那样。