The abundance of Bullet-groups in LCDM
  We estimate the expected distribution of displacements between the two
dominant dark matter (DM) peaks (DM-DM displacements) and between DM and
gaseous baryon peak (DM-gas displacements) in dark matter halos with masses
larger than $10^{13}$ Msun/h. We use as a benchmark the observation of SL2S
J08544-0121, which is the lowest mass system ($1.0\times 10^{14}$ Msun/h)
observed so far featuring a bi-modal dark matter distribution with a dislocated
gas component. We find that $(50 \pm 10)$% of the dark matter halos with
circular velocities in the range 300 km/s to 700 km/s (groups) show DM-DM
displacements equal or larger than $186 \pm 30$ kpc/h as observed in SL2S
J08544-0121. For dark matter halos with circular velocities larger than 700
km/s (clusters) this fraction rises to 70 $\pm$ 10%. Using the same simulation
we estimate the DM-gas displacements and find that 0.1 to 1.0% of the groups
should present separations equal or larger than $87\pm 14$kpc/h corresponding
to our observational benchmark; for clusters this fraction rises to (7 $\pm$
3)%, consistent with previous studies of dark matter to baryon separations.
Considering both constraints on the DM-DM and DM-gas displacements we find that
the number density of groups similar to SL2S J08544-0121 is $\sim 6.0\times
10^{-7}$ Mpc$^{-3}$, three times larger than the estimated value for clusters.
These results open up the possibility for a new statistical test of LCDM by
looking for DM-gas displacements in low mass clusters and groups.
