The dark matter halo shape of edge-on disk galaxies (papers I-IV)

What is the 3D shape of dark matter halos? They are often assumed to be spherical around galaxies, while cosmological simulations form ellipsoids with significant flattening. In this series of 4 papers, the authors study the 21cm HI emission in edge-on galaxies to derive constraints on this flattening. The method is to measure the gas kinematics to derive the forces in the plane, one hand, and the HI thickness to derive the forces perpendicular to the plane, on the other. The HI velocity dispersion that is not known in this direction is assumed constant. This method requires a large amount of modeling to derive the rotation in the plane, to obtain the radial forces. The HI flaring, or progressive radial increase of the thickness of the HI layer, gives the force perpendicular to the plane. Although strong bars might perturb the dynamics in the inner parts, the forces are easier to derive in the outer parts. In the 4th paper, the method and models are carefully applied to the galaxy UGC 7321, and the result shows that its dark halo is spherical. This conclusion is discussed in relation to the assumed vertical distribution of the HI velocity dispersion.