Dynamical models of NGC 3379

MPE   OPINAS   Dynamics Group

The presence of dark matter (DM) has long been inferred around spiral galaxies from their flat rotation curves, and galaxies are now generally believed to be surrounded by extended dark matter halos. Indeed, in the current Λ-cold dark matter (ΛCDM) cosmology, galaxies form within the potential wells of their halos. The standard picture for the formation of elliptical galaxies is through mergers of smaller units. Ellipticals should thus also be surrounded by dark matter halos. Their halos are particularly interesting because ellipticals are among the oldest galaxies and are found in the densest environments. In light of this, the finding of Romanowsky et al. (2003) and Douglas et al. (2007), that several intermediate luminosity ellipticals (NGC 3379, NGC 4494, NGC 821) apparently have only diffuse dark matter halos if any, is quite surprising. The issue is important enough to merit a further careful analysis.


Figure 1: The radially falling PNe velocity dispersion profile from Romanowsky et al. (2003), interpreted by them as indicating a lack of dark matter in NGC 3379. The dotted line indicates a simple model without dark matter.

We have therefore constructed dynamical models of NGC 3379 making use of a variety of kinematic data, including SAURON integral-field data, slit kinematics, and the PN dispersion profile. A schematic view of these data is given in Figure 2.

N3379 obs.

Figure 2: Shows a typical set of observables as used to construct NMAGIC dynamical models of NGC 3379. The kinematic observations consist of integral field SAURON kinematic data (Shapiro et al. 2006) extending to ∼ 1/2-1 Re, slit kinematic data (Statler & Smecker-Hane 1999) out to ∼ 1 1/2 Re and planetary nebulae (PNe) data (Douglas et al. 2007) out to ∼7 Re, which allow to probe the outer halo. The luminosity distribution of the dynamical model is constrained from surface brightness photometric data (Capaccioli et al. 1990, Gebhardt et al. 2000). Often the surface brightness (SB) data are deprojected under some symmetry assumptions to obtain a luminosity density. For deprojecting the SB profile, we use Magorrian's (1999) deprojection method to obtain an axisymmetric intrinsic luminosity distribution.

The modeling results are summarized in Figure 3. We find that the PNe dispersion profile is consistent with range of DM halos. Formal likelihood limits would exclude (at 1σ) the stars only model, as well as models with vcirc(7Re) > 250 km/s. NGC 3379 may well have a DM halo as predicted by merger models from Dekel et al. (2005), provided its outer envelope is strongly radially anisotropic. See De Lorenzi et al. (2009) for a detailed discussion.

vc LH int kin

Figure 3: Summary of the modeling results. The red data point corresponds to the best model. The shaded areas show the results obtained by the merger models discussed by Dekel et al. (2005). Left: Circular velocity curves for the potentials used in the dynamical modelling, including the self-consistent stars-only model and models including different spherical dark matter halos in addition to the stellar component (solid lines). Middle: Relative likelihoods as a function of the model circular velocity at 7Re. Right: Intrinsic kinematics of the final models. The anisotropy profile is shown in the bottom panel. The models are more anisotropic at large radii than the merger simulations from Dekel et al. (2005).