X-ray images highlight galaxy collisions

Viewed in visible light, the elliptical galaxy NGC 4261 looks positively sedate. But a new X-ray image reveals evidence of a violent past–a trail more than 50 light-years across and rife with black holes and neutron stars. The trail indicates that the galaxy collided with another galaxy a few billion years ago, says Lars Hernquist of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

HIDDEN STRUCTURE. X-ray image of the galaxy NGC 4261 reveals a trail of black holes and neutron stars (dots) that can’t be seen in visible light. A. Zezas, et al./CXC/ NASA

He and his colleagues say that the image, recorded by NASA’s Chandra X-ray Observatory, suggests that X-ray studies may be the best way to identify ancient collisions between galaxies. The researchers report their findings in the Dec. 20, 2003 Astrophysical Journal Letters.

The black holes and neutron stars are strung out along the outskirts of the galaxy, which lies 100 million light-years from Earth. These strong X-ray emitters appear to have formed following the demise of a smaller galaxy that was pulled apart as it fell into NGC 4261. The stars formed from the compression of gas in the collision’s aftermath. The ancient collisions fit a model in which the large, elliptical galaxies seen today were formed by the merger of relatively small spiral galaxies in the distant past.


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