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.
