The abundance of bullet groups in ΛcDM

Abstract

We estimate the expected distribution of displacements between the two dominant dark matter (DM) peaks (DM-DM displacements) and between the DM and gaseous baryon peak (DM-gas displacements) in DM halos with masses larger than 1013 h -1 M ⊕. As a benchmark, we use the observation of SL2S J08544-0121, which is the lowest mass system (1.0 × 1014 h -1 M ⊕) observed so far, featuring a bi-modal DM distribution with a dislocated gas component. We find that (50 ± 10)% of the DM halos with circular velocities in the range 300-700 km s-1 (groups) show DM-DM displacements equal to or larger than 186 ± 30 h -1 kpc as observed in SL2S J08544-0121. For DM halos with circular velocities larger than 700 km s-1 (clusters) this fraction rises to (70 ± 10)%. Using the same simulation, we estimate the DM-gas displacements and find that 0.1%-1.0% of the groups should present separations equal to or larger than 87 ± 14 h -1 kpc, corresponding to our observational benchmark; for clusters, this fraction rises to (7 ± 3)%, consistent with previous studies of DM 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 ∼6.0 × 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 ΛCDM by looking for DM-gas displacements in low mass clusters and groups. © 2014. The American Astronomical Society. All rights reserved.