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.
Scale: Image is 8 x 6.5 arcmin per side.
(Credit: X-ray: NASA/CXC/E.O'Sullivan et al; Optical: Palomar DSS)

Chandra X-ray Image of NGC 4555
NGC 4555 is unusual because it is a large elliptical galaxy that is not part of a larger group or cluster. This allowed researchers to use Chandra data to show that the galaxy is embedded in a cloud of 10-million-degree-Celsius gas. These X-ray observations of NGC 4555 confirm that an isolated, elliptical galaxy can possess a dark matter halo of its own.
Scale: Image is 8 x 6.5 arcmin per side.
(Credit: NASA/CXC/E.O'Sullivan et al.)

2MASS Infrared Image of NGC 4555
This image shows the elliptical galaxy NGC 4555 in infrared light as observed by the 2 Micron All-Sky Survey (2MASS). The field of view is the same as the Chandra image.
Scale: Image is 8 x 6.5 arcmin per side.
(Credit: 2MASS/UMass/IPAC-Caltech/NASA/NSF)

Chandra X-ray Image with Scale Bar
Scalebar = 2 arcmin
(Credit: NASA/CXC/Columbia/C. Scharf et al.)