The Cheshire Cat revealed as a merging galaxy group

Abstract

Dominated by two comparably bright elliptical galaxies and exhibiting several large arcs formed through gravitational lensing, this galaxy group resembles the Cheshire Cat popularised by Lewis Carroll in Alice's Adventures in Wonderland. In my thesis, constrained by the distribution of visible matter and a robust identification of related counterparts among the many multiply-lensed images produced from 7 distinct background lensed galaxies in all, I infer the distribution of Dark Matter (DM) in the Cheshire Cat. I find that the Cheshire Cat cannot contain only one but must contain two distinct DM halos, demonstrating definitively that it actually comprises two galaxy groups - thus explaining the bimodal distribution of radial velocities among its group members. The intragroup X-ray gas peaks between the two DM halos, indicating that this gas has been compressed and shock heated by a collision between the two galaxy groups. Computer simulations led by a collaborator suggest that the Cheshire Cat is likely seen about 100 Myr after the first close passage in a collision that will eventually lead to a merger. Interestingly, the two DM halos are displaced inwards with respect to the two dominating elliptical galaxies, suggesting perhaps that DM has a finite collisional cross section. While more work is needed to check whether this inward displacement is caused by not explicitly including the gas component in our lens modelling, and/or whether gravitational interactions have led to an offset between the group DM halos and their corresponding dominant group galaxies, the simplicity of the Cheshire Cat permits a particularly robust and stringent measure of the collisional cross-section of DM.