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7. THE VANISHING

Romanowsky et al. [38] recently probed the outer regions of NGC 3379 up to 3 Re using the Planetary Nebulae Spectrograph finally concluding that dark matter was not required to reproduce the radial dispersion measurements. This result was severely questioned by Dekel et al. [39], who reminded us of the density/anisotropy degeneracy. They showed that a bias towards radial stellar orbits may be naturally expected at a few effective radii, which would produce a dispersion profile similar to the one observed in NGC 3379 even though the mass profile is dominated by dark matter. A recent, but still preliminary, measurement of the stellar velocity dispersion at ~ 3 Re using the SAURON spectrograph by Weijmans et al. indicates that the bulk stellar population has in fact a rather constant dispersion profile, which would, if confirmed, favour the presence of a significant amount of dark matter, and reveal a significant discrepancy between different tracers.

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