Halo of high temperatures X-ray emitting gas (106 to 10 x 106 K) surrounding most large elliptical galaxies. Although
found predominantly around massive elliptical galaxies, weaker X-ray halos have also been detected around starburst galaxies and more recently around spiral galaxies similar to the Milky Way. The photograph (right) shows the X-ray halo in blue superimposed on a visible light image.
Image source: http://chandra.harvard.edu/photo/2006/n5746.