X-ray/Optical Composite of NGC 4555

Dark matter continues to confound astronomers, as NASA's Chandra X-ray Observatory demonstrated with the detection of an extensive envelope of dark matter around an isolated elliptical galaxy. This discovery conflicts with optical data that suggest a dearth of dark matter around similar galaxies, and raises questions about how galaxies acquire and keep such dark matter halos. (Description from http://chandra.harvard.edu/photo/2004/ngc4555/more.html#ngc4555_comp)

Scale: Image is 8 x 6.5 arcmin per side.

http://chandra.harvard.edu/photo/2004/ngc4555/more.html#ngc4555_comp

(Credit: X-ray: NASA/CXC/E.O'Sullivan et al.; Optical: Palomar DSS)

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