The Shape of Dark Matter Halos

Abstract

Techniques for inferring the radial and geometric form of dark matter halos and the results they have produced to date are reviewed. Dark halos appear to extend to at least ~50 kpc with total enclosed masses that rise linearly with radius R. Whether this behavior can be extrapolated to distances as large as 200 kpc and beyond is controversial; results at this radius are model-dependent. Observationally, the geometrical form of the dark halo can be characterized by the equatorial axis ratio b/a (ovalness) and vertical-to-equatorial axis ratio c/a (flattening) of the total density. Different techniques consistently yield b/a > 0.7 (and thus b/a > 0.9 for the potential) at R~20 kpc, with more axisymmetric values, b/a >~ 0.8, being more likely. Results are less consistent for the vertical flattening, perhaps due to the difference in the spatial regions probed by different techniques or inappropriate assumptions. Techniques that probe furthest from the stellar plane z~15 kpc consistently implicate substantially flattened c/a = 0.5 +/- 0.2 dark halos. These axis ratios are in acceptable agreement with expectations from N-body simulations of cold dark matter mixed with ~10% dissipational gas.