Mark Walker, Mark Wardle, PASA, 16 (3), 262.
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Title/Abstract Page: The Cloudy Universe
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Extreme Scattering Events
``Extreme Scattering Events'' (ESEs) were discovered more than a decade ago (Fiedler et al 1987), during a multi-year program to monitor the positions and radio fluxes of a large number of compact quasars. The events themselves amount to large, frequency-dependent variations in the received radio signal, lasting for a month or two. These variations were immediately recognised as a Galactic lensing phenomenon (Fiedler et al 1987; Romani, Blandford & Cordes 1987), rather than being attributable to intrinsic changes in the source. Each lens is in this case just a localised over-density of ionised gas, which refracts the radio-waves, and flux variations occur when a lens crosses the line-of-sight. The lenses are inferred to be a few AU in radius. In the decade following their discovery there was little progress in understanding the physical nature of these lenses; the possibility of generating high electron densities within interstellar shock waves had been explored (e.g. Clegg, Chernoff & Cordes 1988), but without much success in reproducing the observed light-curves. A particular difficulty with the physics of the early models is that the pressure of the ionised gas is a thousand times larger than that of the diffuse interstellar medium, so the latter cannot confine the former and such a lens ought to explode, lasting only a year or so. In order to avoid this, while still maintaining a static lens, one is forced to resort to some contrivance with magnetic fields or highly elongated lenses.Motivated by a desire for a simpler explanation, we investigated a specific physical picture for the lenses in which the hot gas is not static but, rather, forms a continuous outflowing wind (Walker & Wardle 1998a). In turn this implies a reservoir of neutral material at the base of the wind, and to ensure longevity of the lens we assumed this (cold) neutral cloud to be in hydrostatic equilibrium, with its thermal pressure balanced by self-gravity. This turns out to offer a good model for the ESEs: a cold, neutral gas cloud in the Galactic halo is expected to develop a photo-evaporated wind as a consequence of the ionising radiation field arising from hot stars in the Galactic disk (Dyson 1968; see also Henriksen & Widrow 1995); the intensity of this radiation naturally generates the requisite density of ionised gas. Moreover the computed light curves, both at low and high radio frequencies, readily reproduce those of the archetypal ESE in the source 0954+658. Indeed as a model for the ESE phenomenon there seem to be no real difficulties with this picture.
However, it follows from the ESE rate (Fiedler et al 1994), in the context of this physical model, that the neutral gas clouds constitute a large fraction of the mass of the Galaxy. In other words, since they are not present in our current inventories of visible matter, they are a major component of dark matter. This conclusion immediately prompts concerns: is this model consistent with other data? What about the constraints from Big Bang nucleosynthesis? How could such clouds form? How can they survive for so long? The next sections address these issues.
Next Section: Compatibility with other data
Title/Abstract Page: The Cloudy Universe
Previous Section: Identification of dark matter
© Copyright Astronomical Society of Australia 1997