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 [19]. 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
[20],
give remnants that morphologically and
kinematically resemble elliptical galaxies in many ways
[21].
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
[21],
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
[22].
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 /
[23].
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
[18]. 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.