2012ApJS..203...17R

Motivated by a new wave of kinematical tracers in the outer regions of early-type galaxies (ellipticals and lenticulars), we re-examine the role of angular momentum in galaxies of all types. We present new methods for quantifying the specific angular momentum j, focusing mainly on the more challenging case of early-type galaxies, in order to derive firm empirical relations between stellar j_{sstarf} and mass M_{sstarf}(thus extending earlier work by Fall). We carry out detailed analyses of eight galaxies with kinematical data extending as far out as 10 effective radii, and find that data at two effective radii are generally sufficient to estimate total j _{sstarf}reliably. Our results contravene suggestions that ellipticals could harbor large reservoirs of hidden j_{sstarf} in their outer regions owing to angular momentum transport in major mergers. We then carry out a comprehensive analysis of extended kinematic data from the literature for a sample of ∼100 nearby bright galaxies of all types, placing them on a diagram of j_{sstarf} versus M_{sstarf}. The ellipticals and spirals form two parallel j_{sstarf}-M_{sstarf}tracks, with log-slopes of ∼0.6, which for the spirals are closely related to the Tully-Fisher relation, but for the ellipticals derives from a remarkable conspiracy between masses, sizes, and rotation velocities. The ellipticals contain less angular momentum on average than spirals of equal mass, with the quantitative disparity depending on the adopted K-band stellar mass-to-light ratios of the galaxies: it is a factor of ∼3-4 if mass-to-light ratio variations are neglected for simplicity, and ∼7 if they are included. We decompose the spirals into disks and bulges and find that these subcomponents follow j_{sstarf}-M_{sstarf}trends similar to the overall ones for spirals and ellipticals. The lenticulars have an intermediate trend, and we propose that the morphological types of galaxies reflect disk and bulge subcomponents that follow separate, fundamental j_{sstarf}-M_{sstarf} scaling relations. This provides a physical motivation for characterizing galaxies most basically with two parameters: mass and bulge-to-disk ratio. Next, in an approach complementary to numerical simulations, we construct idealized models of angular momentum content in a cosmological context, using estimates of dark matter halo spin and mass from theoretical and empirical studies. We find that the width of the halo spin distribution cannot account for the differences between spiral and elliptical j_{sstarf}, but that the observations are reproduced well if these galaxies simply retained different fractions of their initial j complement (∼60% and ∼10%, respectively). We consider various physical mechanisms for the simultaneous evolution of j_{sstarf} and M_{sstarf}(including outflows, stripping, collapse bias, and merging), emphasizing that the vector sum of all such processes must produce the observed j_{sstarf}-M_{sstarf}relations. We suggest that a combination of early collapse and multiple mergers (major or minor) may account naturally for the trend for ellipticals. More generally, the observed variations in angular momentum represent simple but fundamental constraints for any model of galaxy formation.