Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-10-18
Nature 443:832-834,2006
Astronomy and Astrophysics
Astrophysics
Published in Nature, October 19th issue. Higher resolution figures available at http://aramis.obspm.fr/~bournaud/M31_block_hig
Scientific paper
10.1038/nature05184
The unusual morphology of the Andromeda Spiral (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, M~31 has a well-known outer ring of star formation at a radius of 10 kpc whose center is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped as seen at both optical and radio wavelengths. The halo contains numerous loops and ripples. Here we report the discovery, based on analysis of previously-obtained data, of a second, inner dust ring with projected dimensions 1.5 by 1 kpc and offset by ~0.5kpc from the center of the galaxy. The two rings appear to be density waves propagating in the disk. Numerical simulations offer a completely new interpretation for the morphology of M31: both rings result from a companion galaxy plunging head-on through the center 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.
Ashby Matt L. N.
Barmby Pauline
Block David L.
Bournaud Frederic
Combes François
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