Decaying Neutrinos and the Flattening of the Galactic Halo

The recently constructed Dehnen-Binney set of mass models for the Galaxy is used to show that the decaying neutrino theory for the ionisation of the interstellar medium (Sciama 1990a, 1993) requires the neutrino halo of the Galaxy to be as flattened as is observationally permitted (axial ratio q = 0.2 or shape E8). The argument involves an evaluation of the contribution of red-shifted decay photons from the cosmological distribution of neutrinos to the extragalactic diffuse background at 1500Å. This contribution must be as large as is observationally permitted. These two requirements depend on the decay lifetime τ in potentially conflicting ways. For consistency to be achieved τ must lie within 30 per cent of 10 seconds.