INTRODUCTION

In this talk I will discuss the hypothesis (Sciama 1990a) that most of the dark matter in the Milky Way consists of tau neutrinos whose decay into photons is mainly responsible for the widespread ionisation of hydrogen in the interstellar medium (outside HII regions). I introduced this hypothesis because there are several difficulties with the conventional explanation of the observed ionisation. This explanation involves photons emitted by O and B stars, supernovae etc. The two most important difficulties involve the large opacity of the interstellar medium to ionising photons and the large scale—height of the free electron density. The opacity arises mainly from the widespread distribution of atomic hydrogen in the interstellar medium, which makes it difficult for the ionising photons emitted by widely separated sources to reach the regions where the ionisation is observed. The scale—height of the electron density (as derived from pulsar dispersion measure data by Reynolds (1991)) is about 1 kpc, whereas the scale height of the conventional sources is only about one tenth of this.

Both of these problems would be immediately solved by my neutrino hypothesis since the neutrinos would be smoothly distributed throughout the interstellar medium and their scale—height would be expected to exceed 1 kpc.