We show that the stellar specific angular momentum j, mass M, and bulge fraction of normal galaxies of all morphological types are consistent with a simple model based on a linear superposition of independent disks and bulges. In this model, disks and bulges follow scaling relations of the form and with but offset from each other by a factor of 8 ± 2 over the mass range . Separate fits for disks and bulges alone give and , respectively. This model correctly predicts that galaxies follow a curved 2D surface in the 3D space of , , and . We find no statistically significant indication that galaxies with classical and pseudo bulges follow different relations in this space, although some differences are permitted within the observed scatter and the inherent uncertainties in decomposing galaxies into disks and bulges. As a byproduct of this analysis, we show that the j–M scaling relations for disk-dominated galaxies from several previous studies are in excellent agreement with each other. In addition, we resolve some conflicting claims about the dependence of the j–M scaling relations. The results presented here reinforce and extend our earlier suggestion that the distribution of galaxies with different in the j–M diagram constitutes an objective, physically motivated alternative to subjective classification schemes such as the Hubble sequence.
S. Michael Fall and Aaron Romanowsky. "Angular Momentum and Galaxy Formation Revisited: Scaling Relations for Disks and Bulges" The Astrophysical Journal (2018): 1-13. doi:10.3847/1538-4357/aaeb27