With substantial PN data sets for five galaxies, we can first look at their rotational characteristics. It has long been apparent that the central parts of ellipticals are much less rotationally-dominated than are spirals (e.g., Fall 1983), but there may be large amounts of angular momentum stored in ellipticals' unobserved outer parts — perhaps even more than in spirals because of more dominant major merger histories (J. Primack, this volume). Such high outer rotation is seen in the GCs around M87, but it's not clear that these trace the properties of the dark matter and the main stellar population.
None of the galaxies shows a strong increase in rotation with radius (note though that our sample roundness criterion may introduce a bias toward low spins). Within ∼ 3–5 Reff, they have spin parameters λ' ∼ 0.03–0.07, and at these radii rotation appears dynamically unimportant: v / σ ∼ 0.1–0.3. This conflicts with major merger simulations that predict rapid outer rotation: v / σ ≳ 1 outside 2 Reff (see Fig. 1, left; Weil & Hernquist 1996; Bendo & Barnes 2000).
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Figure 1. Left: Rotational parameter as a function of projected radius for five galaxies observed with PNe (solid lines), and for a typical simulated merger remnant (dotted lines; Weil & Hernquist 1996). Right: Specific angular momentum as a function of enclosed mass, after Fall (1983). Dashed lines outline observations for spiral disks and elliptical's central parts. Dotted lines show CDM hydrodynamical simulations for stellar components (Navarro et al. 1995; Eke et al. 2000). Solid lines show a run with radius for the PN galaxies, where the 1-σ uncertainties are illustrated for one case by dot-dashed lines. |
The ideal comparison is to simulations of galaxy formation in the full cosmological context, but treatments of key baryonic processes such as star formation and feedback are still maturing. Current CDM models do produce low specific angular momenta similar to our observed ellipticals (see Fig. 1, right).