Collisional baryonic dark matter haloes

If dark haloes are composed of dense gas clouds, as has recently been inferred, then collisions between clouds lead to galaxy evolution. Collisions introduce a core in an initially singular dark matter distribution, and can thus help to reconcile scale‐free initial conditions – such as are found in simulations – with observed haloes, which have cores. A pseudo‐Tully–Fisher relation, between halo circular speed and visible mass (not luminosity), emerges naturally from the model: M vis ∝ V 7/2 . Published data conform astonishingly well to this theoretical prediction. For our sample of galaxies, the mass–velocity relationship has much less scatter than the Tully–Fisher relation, and holds as well for dwarf galaxies (where diffuse gas makes a sizeable contribution to the total visible mass) as it does for giants. It seems very likely that this visible‐mass/velocity relationship is the underlying physical basis for the Tully–Fisher relation, and this discovery in turn suggests that the dark matter is both baryonic and collisional.