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J.N. Bregman, J.A. Irwin (University of Michigan)
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 106-107 K and emit X-rays that can constitute 10-30% of 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 in a region defined by the optical extinction and the HI, we detect a shadow in the 0.4-1.0 keV band at the 98% confidence level. This shadow corresponds to approximately 20% of the total point source plus diffuse X-ray background in this energy range, and is consistent with predictions.