Abstract
The soft X-ray background appears to derive from a large but irregularly shaped cavity filled with 10 6 K gas having p/k ≈ 10 4 cm −3 K. This pressure is about the lowest equilibrium value consistent with the weight of the overlying interstellar material. A model for the geometry of the X-ray emitting region is presented for comparison with cavity dimensions established by other means. Under an assumption of approximate equilibrium and relatively normal elemental abundances, a number of timescales and other parameters are evaluated. The picture that emerges is that the large cavity in the local HI distribution was created, superbubble-like, about 2 × 10 7 years ago. The observed hot gas may survive from that time, but could have been reheated by subsequent supernovae within the cavity, with the latest as recent as 1.5 × 10 5 years ago. Within this picture a number of processes are potentially significant: thermal conduction to cavity boundaries providing surface brightened emission; Rayleigh-Taylor instability between the hot gas and overlying HI; acceleration of the observed cosmic rays during the cavity formation; persistence of unthermalized SN ejecta, contributing to ionization at the dense walls; and contamination of material now found within the cavity by stellar mass return. The Local Fluff could even be the aged ejecta of a planetary nebula.
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