One of the most interesting early results obtained with PNe was the discovery of rapid rotation in the halos of the giant elliptical galaxies Centaurus A, NGC 1399, and NGC 1316 [18, 14]. The PNe radial velocity fields in these early-type galaxies show that their outer halos are rapidly rotating, and that these galaxies contain comparable angular momentum to spiral galaxies of similar luminosity. Recent work on several further ellipticals has shown that most of these do not rotate rapidly at large radii; see diagram in . There appears to be a range of outer rotation properties observed.
The amount of rotation in the halos of elliptical galaxies is a valuable probe for how these systems formed. Elliptical galaxies are now believed in general terms to form by merging processes; what is less clear is the kind of progenitors that dominated in the formation of the present population of ellipticals. Spiral-spiral galaxy mergers, observed in the local universe and spurring Toomre's original merger hypothesis , give remnants that morphologically and kinematically resemble elliptical galaxies in many ways . Equal-mass mergers have low v / ~ 0 - 0.2 within R Re, as observed for giant ellipticals, due to angular momentum transfer from inner regions to the extended outer halos by dynamical friction in the merger. These remnants can, however, contain significant angular momentum at large radii, reaching v / ~ 0.2 - 0.5, even though a lot of spin angular momentum is carried away by material in the tidal tails. Unequal-mass mergers rotate faster than equal-mass mergers. As argued in , binary mergers of disk galaxies may be the main formation mechanism of low- and intermediate mass ellipticals.
An alternative merging channel to form an elliptical is through multiple major and minor mergers in a compact group of galaxies . In this case the tidal forces are more effective in disrupting the progenitors before coalescence, so dynamical friction is less effective. As a result, the remnants have more angular momentum in their inner parts than spiral-spiral mergers, placing them not far from the oblate-isotropic line in the v / - diagram, and their outer parts may reach v / ~ 1.
Computations of the angular momentum of dark matter halos growing by merging and accretion in hierarchical universes result in low spin parameter and low v / . Interestingly, the values of these parameters in the evolution of individual halos are most likely to increase in major mergers, and generally decrease in multiple accretion of satellites. If this is indicative for the luminous components also, then ellipticals that were last shaped by a major merger should contain the highest angular momenta. The rotation velocities in the remnant halos are fairly constant with radius, however, confirming that the dissipation and dynamical friction processes acting specifically on the baryonic component are crucial for shaping the angular momentum distributions in elliptical galaxies.
These results show that there is no simple, one-to-one correspondence between angular momentum at large radii and formation mechanism. For example, of the ellipticals with outer PN kinematics, Centaurus A is believed to have formed from the merger of two disk galaxies; in this galaxy v / rises to ~ 1 beyond R = 15 kpc . However, how much angular momentum resides in the outer halos of elliptical galaxies is clearly a key issue which, when understood for a representative sample of elliptical galaxies, will be crucial for determining the merging channel that dominated their formation. This is a research program that can ideally be tackled by PNe radial velocity measurements with the PN.S.