A shadow of the extragalactic X-ray background

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 10(5)-10(7) 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.