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An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy

Galaxies go head-to-head The Andromeda galaxy (Messier 31) is big in galaxy evolution studies, since it is the nearest giant spiral galaxy, and one of the largest in the Local Group. It also looks odd, and its appearance has long puzzled astronomers. It has a well known outer ring offset from the ga...

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Bibliographic Details
Published in:Nature 2006-10, Vol.443 (7113), p.832-834
Main Authors: Block, D. L., Bournaud, F., Combes, F., Groess, R., Barmby, P., Ashby, M. L. N., Fazio, G. G., Pahre, M. A., Willner, S. P.
Format: Article
Language:English
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Summary:Galaxies go head-to-head The Andromeda galaxy (Messier 31) is big in galaxy evolution studies, since it is the nearest giant spiral galaxy, and one of the largest in the Local Group. It also looks odd, and its appearance has long puzzled astronomers. It has a well known outer ring offset from the galactic nucleus, and the outer disk is warped. Now an offset inner dust ring has been discovered in infrared images from the Spitzer Space Telescope. Numerical simulations indicate that both rings formed when a companion galaxy plunged almost head-on through the disk of M31. The most likely interloper is the dwarf galaxy M32. This paper reports the presence of a second, inner ring offset by half a kiloparsec from the centre of the Andromeda galaxy. Numerical simulations indicate that both rings result from a companion galaxy plunging head-on through the centre of the disk of M31. The unusual morphology of the Andromeda galaxy (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M31 has a well-known 1 , 2 , 3 , 4 , 5 , 6 , 7 outer ring of star formation at a radius of ten kiloparsecs whose centre is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped, as seen at both optical 8 and radio 9 wavelengths. The halo contains numerous loops and ripples. Here we report the presence of a second, inner dust ring with projected dimensions of 1.5 × 1 kiloparsecs and offset by about half a kiloparsec from the centre of the galaxy (based upon an analysis of previously-obtained data 10 ). The two rings appear to be density waves propagating in the disk. Numerical simulations indicate that both rings result from a companion galaxy plunging through the centre of the disk of M31. The most likely interloper is M32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.
ISSN:0028-0836
1476-4687
1476-4679
DOI:10.1038/nature05184