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

  title={An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy},
  author={David L. Block and Fr{\'e}d{\'e}ric Bournaud and Françoise Combes and Robert Groess and Pauline Barmby and Matthew L. N. Ashby and Giovanni G. Fazio and Michael Andrew Pahre and Steven P. Willner},
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 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 and radio wavelengths. The halo contains numerous loops and ripples. Here we report the… 
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