2002ApJ...565L..13B

The majority of baryons in the local universe are expected to lie outside of galaxies, in modest overdensity regions of the universe. This gas is predicted to be at a temperature of 10⁵-10⁷ K and to emit X-rays that can contribute significantly to the X-ray background near 0.75 keV. If present, the gas in an edge-on galaxy would absorb this diffuse background emission, causing a shadow. We carried out this test using Chandra observations of the edge-on galaxy NGC 891 and using a cool, gas-rich region defined by high optical extinction, and we detect a shadow in the 0.4-1.0 keV band at the 99% confidence level. This shadow corresponds to approximately one-half of the total X-ray background in this energy range, brighter than the mean predicted value for emission from diffuse baryons but within the range of model values.