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RE: NGC 922
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Hubble Sees a Galaxy Hit a Bull's-Eye

Bright pink nebulae almost completely encircle a spiral galaxy in this NASA/ESA Hubble Space Telescope image of NGC 922. The ring structure and the galaxy's distorted spiral shape result from a smaller galaxy scoring a cosmic bull's-eye, hitting the center of NGC 922 some 330 million years ago. Hubble's image of NGC 922 consists of a series of exposures taken in visible light with the Wide Field Camera 3, and in visible and near-infrared light with the Wide Field and Planetary Camera 2.
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Title: Chandra Observations of the Collisional Ring Galaxy NGC 922
Authors: A. H. Prestwich, J. L. Galache. T. Linden, V. Kalogara, A. Zezas, T. P. Roberts, R. Kilgard, A. Wolter, G. Trinchieri

In this paper we report on Chandra observations of the starburst galaxy NGC 922. NGC 922 is a drop-through ring galaxy with an expanding ring of star formation, similar in many respects to the Cartwheel galaxy. The Cartwheel galaxy is famous for hosting 12 ULX, most of which are in the star forming ring. This is the largest number of ULX seen in a single system, and has led to speculation that the low metallicity of the Cartwheel (0.3 solar) may optimise the conditions for ULX formation. In contrast, NGC 922 has metallicity near solar. The Chandra observations reveal a population of bright X-ray sources, including 7 ULX. The number of ULX in NGC 922 and the Cartwheel scales with the star formation rate: we do not find any evidence for an excess of sources in the Cartwheel. Simulations of the binary population in these galaxies suggest that the ULX population in both systems is dominated by systems with strong wind accretion from supergiant donors onto direct-collapse BHs. The simulations correctly predict the ratio of the number of sources in NGC 922 and the Cartwheel. Thus it would appear that the the metallicity of the Cartwheel is not low enough to see a difference in the ULX population compared to NGC 922.

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