Smoking gun evidence of a head-on collision for Andromeda
1er octobre 2006
The structure of the disk of Andromeda (or Messier 31), the largest spiral galaxy in the Local Group, has always been a puzzle : the map of interstellar medium is dominated by a ring, which appears superposed to pieces of spiral arms. The discovery by the infrared satellite Spitzer of a second ring at the center of the galaxy, allowed an international team of researchers (including two scientists from Paris Observatory, LERMA, UMR-CNRS) to propose a solution to the problem, thanks to numerical simulations : both rings would be density waves propagating from the center, just after the impact of a small companion galaxy, which would have crossed the Andromeda disk. Results are published this week in Nature.
Andromeda, the nearest spiral galaxy of our Milky Way, is still concealing a lot of riddles. One of these is the dust and gas morphology and astronomers are trying to solve this problem since twenty years. Indeed, how to explain a warped ring whose center is offset by 0.5 kpc from the galaxy nucleus, and seen at both optical and radio wavelengths in the outer region of Andromeda ? Thanks to near and mid-infrared images provided by the Spitzer Space Telescope, a scientific international team found the solution.
Figure 1 : Emission of warm dust grains and macromolecules in the interstellar medium of the Andromeda Galaxy M 31 observed with the Infrared Array Camera on board the Spitzer Space Telescope. M 31 clearly possesses two rings. Apart from the famous outer dust ring seen at a radius of 10 kpc (underlined here in green), this map reveals a second 1.5 by 1 kpc inner dust ring (indicated here in blue), offset by approximately 0.5 kpc from the galaxy nucleus. Both rings are interpreted to be density waves induced by an almost head-on collision. The most likely candidate is the dwarf companion galaxy M 32, which appears faint in this image because it has little dust.
Spitzer gives a special clue which was invisible until now : a second ring constituted of dust and gas inside M 31 and smaller than the previous one. This one is located in the inner region and its center is also offset by 0.5 kpc from the galaxy nucleus. Actually, this ring would be the second ripple of a wave propagating from the center of the galaxy to the outer region and caused by a huge event, just like a stone thrown in the water. At this galactic scale, only a collision between M 31 and a companion could explain these two rings, both of them off-centered from the galactic nucleus. If so, who is the culprit ? The international scientific team, with numerical simulations, shows that the nearest neighbour of Andromeda, the galaxy M 32, would be a good candidate. The growth, the mass and the distance between M 32 and M 31, all these clues are totally compatible with the hypothesis of a collision between the two galaxies. M 32 can not hide anymore, the culprit has been found !
Figure 2 : Gas morphology produced by simulations of a head-on encounter between M 31 and M 32. These N-body models include the gravitational dynamics of stars, dark matter, the dissipation of gas and the formation of stars. The dashed red line demarcates the orbit of M 32 ; the locale of impact lies very close to the polar axis of M 31. Snapshots a, b, and c occur at t= 35 million years (Myr) prior to collision and at 100 and 210 Myr post impact ; the latter snapshot also shows the position of M 32 as we see it today. All snapshots are with the disk viewed at an inclination angle of 77 degrees for direct comparison with Figure 1. Snapshot c shows the central region of M 31 warped by a tilt angle of 30 degrees with respect to the main disk plane in accord with observations. The two ring-like waves (both offset from the galaxy center) are seen, as is the hole in the outer ring. Snapshot d shows the gas morphology at t=210 Myr starting from the same initial disk conditions but modeled without a collision ; no density-wave rings are generated. The initial mass ratio for the companion M 32 is 1/10th that of M 31 (or 1/13th excluding dark matter). M 32 would now be located at a distance of 35 kpc and at a galactic latitude of about 45 degrees, which is fully compatible with its observed position.
Moreover, as Andromeda is located in the neighbourhood of our galaxy, this discovery is a great opportunity to study more precisely the consequences of galaxy collisions.
Movie : Development of ring density waves in the spiral disk of Andromeda.
Reference : An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy Block D.L.(1), Bournaud F.(2,3), Combes F.(2), Groess R.(1), Barmby P.(4), Ashby M.L.N.(4), Fazio G.G.(4), Pahre M.A.(4), Willner S.P.(4) 2006, Nature, 19th October, http://arxiv.org/abs/astro-ph/0610543 (1) Anglo American Cosmic Dust Laboratory, School of Computational and Applied Mathematics, University of the Witwatersrand, Private Bag 3, WITS, 2050, South Africa (2) Observatoire de Paris, LERMA, 61 Av. de l’Observatoire, F-75014 Paris, France (3) SAp-CEA, Orme des Merisiers, F-91191 Gif-sur-Yvette, France (4)Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts 02138, USA Press release written by Cyrille Baudouin, with the support of SF2A (Société Française d’Astronomie et d’Astrophysique) See also : the press release from C.N.R.S. the press release from C.E.A. that of Center for Astrophysics, Harvard
Contact Frédéric Bournaud (Observatoire de Paris, LERMA, et CEA) Francoise Combes (Observatoire de Paris, LERMA, et CNRS)
Dernière modification le 4 mars 2013