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


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-known1,2,3,4,5,6,7 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 optical8 and radio9 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 data10). 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.

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Figure 1: The Andromeda galaxy M31 observed with IRAC on board the Spitzer Space Telescope at a wavelength of 8 µm.
Figure 2: Gas morphology produced by simulations of a head-on encounter between M31 and M32.


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This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Funding for this work was provided by the Anglo American Chairman’s Fund as well as by NASA through an award issued by JPL/Caltech. Author Contributions P.B., G.G.F., M.L.N.A., M.A.P. and S.P.W. provided the IRAC data of M31 on which the analyses are based. D.L.B. initiated the collaboration between the USA, France and WITS, South Africa and identified Baade dust spirals in emission in the central region of M31. F.C. discovered the central ring between the inner Baade arms and believed it to be induced by a head-on collision. F.B. performed the numerical simulations which fully corroborated this interpretation and also generated Figs 1 and 2. R.G. cleaned the IRAC images of foreground stars and conducted Fourier spectral analysis of the Baade arms. F.C. and F.B. provided crucial drafts of this Letter. The final revised versions were prepared by D.L.B., S.P.W, R.G. and M.L.N.A.

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Block, D., Bournaud, F., Combes, F. et al. An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy. Nature 443, 832–834 (2006).

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