The thickness of the neutral hydrogen layer , coupled with the rotation curve , traces the outer dark matter potential .
We estimate the amplitude of the flaring in spiral galaxies from a 3D model of the HI gas .
Warps in particular are explicitly parametrized in the form of an harmonical density wave .
Applying our method to the galaxy NGC 891 , the only model that could fit the observations , and in particular the HI at large height above the plane , includes a strong warp with a line of node almost coinciding with the line of sight .
This high-Z HI is not observed at the most extreme velocity channels , those corresponding to high rotational velocities .
This is accounted for by the model , since orbits in the tilted planes are not circular , but elongated , with their minor axis in the galaxy plane .
Their velocity on the major axis ( i.e .
at their maximal height above the plane ) is then 30 % less than in the plane .
We finally connect the modelled vertical outer gaseous distribution to the dark matter through hydrodynamical and gravitational equations .
Under the assumption of isotropy of the gaseous velocity dispersion , we conclude on a very flattened halo geometry for the galaxy NGC 891 ( q \approx 0.2 ) , while a vertical velocity dispersion smaller that the radial one would lead to a less flattened Dark Matter Halo ( q \approx 0.4 - 0.5 ) .
Both results however suggests that dark matter is dissipative or has been strongly influenced by the gas dynamics .