Planetary Nebulae (PNe) are excellent test particles to trace the
kinematics in the outer halos of elliptical galaxies. They occur as a
brief stage in the late evolution of stars with masses between
~ 0.8 - 8
M
, when
these stars evolve from the asymptotic giant
phase to their final white dwarf stage. Their nebular gas envelope
converts up to 15% of the central star's radiation energy to photons
in the characteristic
5007 [OIII] emission
line
[1]. From
this emission line, PNe can be identified and their radial velocities
measured spectroscopically at distances of up to 20-30 Mpc. With 8m
class telescopes
[2]
or the special-purpose Planetary Nebula
Spectrograph (PN.S)
[3],
several hundred PNe have been found in giant
elliptical galaxies with distance up to the Virgo cluster. The PN
number density approximately follows the surface density of stars;
thus a few tens of PNe can be used to measure the stellar kinematics
at several effective radii Re, where the surface
brightness is too faint for absorption line spectroscopy (ALS).
From these PN velocities, constraints on the angular momentum, dynamics, and mass distribution in the outer halos of ellipticals can be derived; these are important to compare with predictions from theories of galaxy formation.
In this analysis, it is always advantageous if additional information can be included. Especially, stellar kinematics from ALS are highly desirable (i) to determine the mass-to-light ratio in the inner, presumably baryon-dominated parts of the galaxy, and (ii) to narrow down the range of permitted dynamical anisotropies at 1-2 effective radii [4]. Also, globular cluster (GC) velocities are now becoming available in substantial numbers around luminous ellipticals. While the spatial distribution and dynamics of the globular cluster system may be different from those of the stars, analysis of these velocities gives additional constraints on the common gravitational potential [5]. Lastly, an ideal case is when the galaxy potential is narrowly constrained from Chandra or XMM X-ray observations [6]. Then the information obtained with the PN kinematics can be used entirely for inferring the orbital structure of the outer stellar halo in the known potential. These issues are further discussed later; the present section is concerned with PNe as kinematic tracers.