A Shadow of the Diffuse X-Ray Background

Joel N. Bregman,Jimmy A. Irwin (University of Michigan)

[Contributed talk, 15min.]


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 10 $^{{\rm 5}}$-10 $^{{\rm 7}}$ K and 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 in a gas-rich region defined by high optical extinction, we detect a shadow in the 0.4-1.0 keV band at the 98% confidence level.  This shadow corresponds to approximately one-third to one-half of the X-ray background in this energy range at this location.  This observation is consistent with theoretical models, which predict a wide range of spatially dependent values for the background.



Himel Ghosh