Angular momentum and galaxy formation revisited
  Motivated by new kinematic data in the outer parts of early-type galaxies
(ETGs), we re-examine angular momentum (AM) in all galaxy types. We present
methods for estimating the specific AM j, focusing on ETGs, to derive relations
between stellar j_* and mass M_* (after Fall 1983). We perform analyses of 8
galaxies out to ~10 R_e, finding that data at 2 R_e are sufficient to estimate
total j_*. Our results contravene suggestions that ellipticals (Es) harbor
large reservoirs of hidden j_* from AM transport in major mergers. We carry out
a j_*-M_* analysis of literature data for ~100 nearby bright galaxies of all
types. The Es and spirals form parallel j_*-M_* tracks, which for spirals is
like the Tully-Fisher relation, but for Es derives from a mass-size-rotation
conspiracy. The Es contain ~3-4 times less AM than equal-mass spirals. We
decompose the spirals into disks+bulges and find similar j_*-M_* trends to
spirals and Es overall. The S0s are intermediate, and we propose that
morphological types reflect disk/bulge subcomponents following separate j_*-M_*
scaling relations -- providing a physical motivation for characterizing
galaxies by mass and bulge/disk ratio. Next, we construct idealized
cosmological models of AM content, using a priori estimates of dark matter halo
spin and mass. We find that the scatter in halo spin cannot explain the
spiral/E j_* differences, but the data are matched if the galaxies retained
different fractions of initial j (~60% and ~10%). We consider physical
mechanisms for j_* and M_* evolution (outflows, stripping, collapse bias,
merging), emphasizing that the vector sum of such processes must produce the
observed j_*-M_* relations. A combination of early collapse and multiple
mergers (major/minor) may account for the trend for Es. More generally, the
observed AM variations represent fundamental constraints for any galaxy
formation model.
