Ionized Hydrogen at Large Galactocentric Distances

J. Bland-Hawthorn, PASA, 14 (1), 64.

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Further Experiments

The edges of spirals are of intrinsic interest for a number of related experiments. As is well known, the MACHO experiment has shown that maybe as much as half the dark matter out to the distance of the LMC is made up of baryons. One possibility is low luminosity white dwarfs. In his book, Sciama (1996) demonstrates that tau neutrinos with masses 24 eV and lifetimes tex2html_wrap_inline353 10tex2html_wrap_inline381 sec explain a gamut of disjoint observations. Sciama neutrinos share an important similarity with white dwarfs as both could, in principle, make up the dark halo and produce sufficient flux to ionize spiral edges (Sciama 1995).

Once weak line emission is detected, it is very difficult to identify the source of ionization unambiguously. Thus, we have chosen to study a diverse class of objects in different environments. In other words, to rule out ram pressure from an external medium, we choose galaxies in loosely bound clusters (e.g. Ursa Major). To rule out internal sources of ionization, we choose ellipticals with extended HI disks (e.g. NGC 5266), but even here there is an uncertain contribution from ``UV upturn'' stars. While the early results are promising, we shall refrain from concluding this review with some general statements.

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