A Study of Abell 2146: Dark and Luminous

The universe is a highly dynamic system. Under the primary influence of gravity, matter in the universe clumped together to create the structure we currently observe with our telescopes. Gravity formed objects on vastly different scales, from planets and stars that form solar systems, to vast collections of star systems that comprise galaxies, and then collections of galaxies to create the largest gravitationally bound objects in the universe, galaxy clusters.

The structure we see in the universe forms hierarchically in the sense that two smaller objects come together under the influence of gravity and create something bigger. Here we study one such merging event in the galaxy cluster Abell 2146. It is a rare system given that it appears that the time since the two galaxy clusters crashed into one another is rather short in the scheme of the universe. Abell 2146 is also positioned at just the orientation in the sky that we can observe the collision from the side. This gives us a great view of the galaxies, the dark matter, and the intervening plasma between the galaxies that give off X-rays.

We localize the position of the dark matter component in the cluster by use of gravitational lensing. We construct models that act as a lens that distorts the light from background galaxies. By looking at how the light is distorted, we reconstruct the location of dark matter in the system in tandem with the regular ordinary matter. This gave the exciting result that the bright peak in the X-ray signal coming from the plasma or intracluster medium was traveling in front of the more massive component of the system, which is unique among the few systems that exist like this.

In an attempt to understand what could have caused this strange configuration, hydrodynamic simulations were performed using simple density profiles as proxies for the dark matter and plasma that make up the bulk of galaxy clusters. Constraints were obtained from previous studies to reduce the explored parameter space. Mock X-ray maps were generated from simulation results and compared to observations from the Chandra X-ray Observatory. We find there must be some additional physics needed that was not implemented in the simulations or some unknown starting condition to match certain features observed in X-ray. We obtained simulation results where we see a bright X-ray peak ahead of the more massive cluster but without shock fronts and also at a much later time in the evolution of the system than what was originally thought.