The information derivable from stellar kinematics at R >> Reff is manifold. One could first of all easily establish the presence or absence of a large dark halo. So far, only a small handful of ellipticals have been studied well enough to verify the presence of dark matter, and those have been exclusively confined to the brightest galaxies. One could also determine in more detail the radial profile
of the mass distribution. This offers a key
test of CDM hierarchical merger models (e.g., Navarro, Frenk & White
1997), which predict a specific functional form for , as well
as a tight correlation between the function's parameters (total mass
and concentration). In the outer parts of the galaxies
(~ 10 kpc), CDM predicts a dark matter density profile
~ r-2.5, while monolithic collapse models predict
~ r-2. Thus, by measuring at large radii, where the
complicating effects of baryonic dissipation are insignificant, we
will be able to make a clean test of the different models of galaxy
formation.
Another readily-determined quantity is the angular momentum of the stars in galaxies' outer parts. Although the main bodies of ellipticals are known to contain much less angular momentum than in spirals of similar luminosity, some simulations of galaxy formation (Zurek et al. 1988; Barnes 1992; Weil & Hernquist 1996) suggest that large amounts of angular momentum resides in the peripheries of these systems. These models are approaching the point of being able to make robust and detailed predictions of the magnitude and direction of angular momentum in ellipticals, and it is important to investigate such properties observationally for a reasonably-sized sample of these galaxies.
The orbital structure of the stars in the outer parts can also provide clues about the formational histories of ellipticals. In the inner parts, the strong effects of baryonic and dynamical dissipation will have likely altered the stellar orbital distribution considerably, but in the outer parts (R ~ 20 kpc ~ 5 Reff), the dynamical times are long enough (~ 1-2 Gyr) to leave some observable ``residue'' of the galaxy's formation (such as recent mergers) in the form of kinematical substructure.
It is clear theoretically that there is much to be learned by studying the outer parts of ellipticals. Furthermore, deep surface photometry of ellipticals has turned up clear evidence that there are often distinct stellar components at large radii (e.g., Schombert 1986; Weil et al. 1997). The nature of these components will only become apparent once their kinematics is measured. One of the most promising techniques for making such measurements is by using planetary nebulae (PNe) as dynamical tracers. PNe are abundant in all galaxy types, and the fact that they emit most of their light at a select few discrete wavelengths means that, with the use of appropriate filters, they can be identified against the background light to many Reff, and that their velocities can be easily measured. Although globular clusters can be used for some of these purposes, PNe are better employed because they are direct tracers of a galaxy's bulk stellar distribution.
Previous observing programmes have examined the kinematics of PNe around a few giant ellipticals using general-purpose instrumentation (see figure below; Ciardullo et al. 1993; Arnaboldi et al. 1994, 1996, 1998; Tremblay et al. 1995; Hui et al. 1995). The results have been tantalising, suggesting that these galaxies have large amounts of angular momentum misaligned with their inner stellar isophotes. Unfortunately, these programmes have been hampered by a number of observational difficulties, as well as by the inordinate amount of time required to acquire PNe in sufficient numbers.
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A sample of 433 planetary nebulae (PNe) overlaid on the stellar isophotes
of NGC 5128,
where Reff  5 kpc.
The PNe marked with circles have negative relative velocities, and
the PNe with crosses have positive velocities.
 marks the position of the rotation axis.
With our proposed observations, we will be able to obtain data of
comparable quality for a much more representative sample of
elliptical galaxies.
From Hui et al. (1995).
(Click on figure to enlarge.)
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With the advent of the Planetary Nebula Spectrograph (PN.S), all these issues may finally be tackled head-on. The PN.S is a specialised instrument, optimised for measuring PN kinematics, which is set to become a World-leading facility for measuring the dynamics of the outer parts of early-type galaxies. The instrument will be complementary to SAURON, the WHT spectrograph currently surveying the inner parts of such galaxies (R < Reff; Bacon et al. 2001). Drawing on these unprecedented dynamical studies, we will finally be able to unravel the nature and origin of early-type galaxies.
For information about our first proposed observing programme with the PN.S, see the next page.
For a comparison of PNe and globular clusters as dynamical tracers, see here.
A further use for observations of PNe in external galaxies is as tertiary distance indicators. The luminosity function (PNLF) of the PNe in any given galaxy has been demonstrated to follow a rather universal functional form. In particular, the PNLF cuts off at the bright end at an absolute magnitude which is relatively invariant among different galaxies.
Just how reliable is the PNLF as a distance indicator? Don't take our word for it, but look at what Robin Ciardullo, one of the best-known workers in the field, has to say (click here). Note that [O III] observations alone do not provide sufficient discrimination against HII regions to prevent contamination of the PNLF. The PNLF method can be applied to spiral galaxies (read this Ap.J. abstract) but only if auxiliary measurements are made. Distance work with the basic PNS is therefore limited to early-type galaxies.
Where do PNe distances fit in with other distance indicators?
Ciardullo provides the following
schematic.
To what distance will reliable results be obtained? The bright end of the PNLF is unambiguously measured when the `flat' portion of the luminosity function can be fitted. This requires completeness down to approximately one magnitude below the brightest PNe (`completeness' here means freedom from flux-dependent selection). According to the simulations done this occurs for D=15 Mpc in 7 hours of observing with a 4-meter-class telescope.