Spiral galaxy NGC 5746. Combined X-ray and optical image of the spiral galaxy NGC 5746. The optical part of this image (central plane of the galaxy) was obtained with a ground-based telescope. The hotter, X-ray emitting region (blue, spherical) was imaged with the Chandra X-ray Observatory (CXO) from Earth orbit. This halo of hot gas around the galaxy extends for 60,000 light years either side. Theories of galaxy formation predict these halos of hot gas, but this observation (published in 2006) is the first time one has been detected around a quiet (non-active) galaxy, thus reinforcing the formation theories. NGC 5746 is 100 million light years from Earth, in the constellation of Virgo.