New Perspectives on Galactic Angular Momentum, Galaxy Formation, and the Hubble Sequence

This paper provides a summary of our recent work on the scaling relations between the specific angular momentum $j_{\star}$ and mass $M_{\star}$ of the stellar parts of normal galaxies of different bulge fraction $\beta_{\star}$. We find that the observations are consistent with a simple model based on a linear superposition of disks and bulges that follow separate scaling relations of the form $j_{\star{\rm d}} \propto M_{\star{\rm d}}^{\alpha}$ and $j_{\star{\rm b}} \propto M_{\star{\rm b}}^{\alpha}$ with $\alpha = 0.67 \pm 0.07$ but offset from each other by a factor of $8 \pm 2$ over the mass range $8.9 \leq \log (M_{\star}/M_{\odot}) \leq 11.8$. This model correctly predicts that galaxies follow a curved 2D surface in the 3D space of $\log j_{\star}$, $\log M_{\star}$, and $\beta_\star$.