Soft X-ray Emission from Intergalactic Gas in the Neighbourhood of the Galaxy

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Abstract
The gravitational influence of our Galaxy on a hypothetical Local Group plasma, which was calculated in a previous paper by one of us (R.H.), is here shown to give rise to a flux of soft X-rays which may already have been detected. The required parameters of the plasma are |$n\sim 2-5\times{10}^{-4}\text{cm}^{-3}\,\text{and}\,T\sim 2-30\times{10}^{5\,\circ}$|K. The predicted soft X-ray flux would vary systematically with the angle between the line of sight and the direction to the galactic centre by a factor depending on T, but of order 4 in typical cases. The fraction of the radiation coming from distances closer than the Magellanic Clouds depends on T, and is calculated to exceed 75 per cent for |$T\lt 7\times{10}^{5\,\circ}$|K. Recent preliminary observations scanning the Small Magellanic Cloud suggest that this fraction does exceed 75 per cent. If the emitting gas contains a small admixture of heavy elements expelled by the Galaxy, soft X-ray lines might be detectable. For example, if the relative abundance of oxygen |$\sim {10}^{-2} $| of its solar value, the combined equivalent width of the 21·6, 21·8 and 22·1 Å lines of O VII would ~ 10 Å for an emitting temperature |$\sim 2\times{10}^{6\,\circ}$|K. Equivalent widths are given for all the lines of possible interest, and their angular variations discussed. Finally the integrated soft X-ray flux from other clusters of galaxies is considered.

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