Extreme Scattering Events and Galactic Dark Matter

Extreme Scattering Events (ESEs) are attributed to radio-wave refraction by acloud of free-electrons crossing the line-of-sight. We present a new model inwhich these electrons form the photo-ionized 'skin' of an underlying cool,self-gravitating cloud in the Galactic halo. In this way we avoid the severeover-pressure problem which afflicts other models. The UV flux in the Galactichalo naturally generates electron densities of the right order. We demonstrate,for the first time, a good reproduction of the prototypical ESE in the quasar0954+658. The neutral clouds are a few AU in radius and have masses less thanabout 10^{-3} solar. The observed rate of ESEs implies that a large fraction ofthe mass of the Galaxy is in this form.Comment: 5 pp incl 3 figs, LaTeX, uses aas2pp4.sty. Minor revisions. ApJ Letters in pres