Dark matter-baryons separation at the lowest mass scale: the Bullet   Group
  We report on the X-ray observation of a strong lensing selected group, SL2S J08544-0121, with a total mass of $2.4 \pm 0.6 \times 10^{14}$ $\rm{M_\odot}$ which revealed a separation of $124\pm20$ kpc between the X-ray emitting collisional gas and the collisionless galaxies and dark matter (DM), traced by strong lensing. This source allows to put an order of magnitude estimate to the upper limit to the interaction cross section of DM of 10 cm$^2$ g$^{-1}$. It is the lowest mass object found to date showing a DM-baryons separation and it reveals that the detection of bullet-like objects is not rare and confined to mergers of massive objects opening the possibility of a statistical detection of DM-baryons separation with future surveys. 
F. Gastaldello, M. Limousin, G. Fo\"ex, R. P. Mu\~noz, T. Verdugo, V.   Motta, A. More, R. Cabanac, D. A. Buote, D. Eckert, ...
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