NGC 891: Shadow Reveals Intergalactic Hot Gas
The optical image (left) of NGC 891 shows a dark disk of dust and gas, with contours outlining the region where the X-ray shadow of the galaxy is darkest. The Chandra image (right) shows a few point-like sources due to neutron stars and black holes, and a shadow cast by the galaxy on an otherwise bright background of X-rays. This shadow is evidence for a source of X-rays beyond the galaxy. These X-rays are thought to come from a vast, distant intergalactic cloud of multimillion degree gas.
(Credit: NASA/CXC/U.Mich./J.Bregman & J.Irwin)

Illustration of Absorption by Intergalactic Gas
In this artist's rendition, X-rays from a distant quasar traveling toward Earth and the Chandra X-ray Observatory are shown as waves of colored light. As they pass through a cloud of intergalactic gas, which appears as the white, filamentary structure, some of the X-rays are absorbed.
(Credit: NASA/CXC/A.Hobart;)

X-ray Spectrum of PKS 2155-304
This figure shows how oxygen ions in the clouds produce dips in the spectrum, or X-ray energy signature, of the quasar PKS 2155-304. A careful study of these dips enables astronomers to estimate the temperature, density, and mass of the absorbing gas.
(Credit: NASA/MIT/T.Fang et al.)

X-ray Image of NGC 891
The Chandra image of NGC 891 shows a few point-like sources due to neutron stars and black holes, and a shadow cast by the galaxy on an otherwise bright background of X-rays. This shadow is evidence for a source of X-rays beyond the galaxy. These X-rays are thought to come from a vast, distant intergalactic cloud of multimillion degree gas.
(Credit: NASA/CXC/U.Mich./J.Bregman & J.Irwin)