Angular Momentum and Galaxy Formation Revisited: Scaling Relations for Disks and Bulges

TL;DR

This paper demonstrates that galaxy stellar angular momentum, mass, and bulge fraction follow predictable scaling relations across galaxy types, supporting a model of combined disks and bulges that explains their properties.

Contribution

It introduces a simple superposition model for disks and bulges that accurately predicts galaxy angular momentum and mass relations across morphological types.

Findings

Disks and bulges follow power-law scaling relations with similar slopes.

Galaxies occupy a curved surface in the space of angular momentum, mass, and bulge fraction.

No significant difference in relations between classical and pseudo bulges.

Abstract

We show that the stellar specific angular momentum j_*, mass M_*, and bulge fraction beta_* 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 j_*d ~ M_*d^alpha and j_*b ~ M_*b^alpha with alpha = 0.67 +/- 0.07 but offset from each other by a factor of 8 +/- 2 over the mass range 8.9 <= log M_*/M_Sun <= 11.8. Separate fits for disks and bulges alone give alpha = 0.58 +/- 0.10 and alpha = 0.83 +/- 0.16, respectively. This model correctly predicts that galaxies follow a curved 2D surface in the 3D space of log j_*, log M_*, and beta_*. 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 beta_*-dependence of the j_*--M_* scaling relations. The results presented here reinforce and extend our earlier suggestion that the distribution of galaxies with different beta_* in the j_*--M_* diagram constitutes an objective, physically motivated alternative to subjective classification schemes such as the Hubble sequence.