A Line-Of-Sight Galaxy Cluster Collision: Simulated X-Ray Observations

TL;DR

This paper uses high-resolution simulations and mock X-ray observations to study a line-of-sight galaxy cluster collision, revealing how projection effects influence observed properties and mass estimates.

Contribution

It presents the first detailed simulation and mock X-ray analysis of a line-of-sight galaxy cluster collision, highlighting the impact on observational interpretation and mass estimation.

Findings

Post-collision profiles fit better with two $eta$-models than one.

Projection effects cause observed temperatures to be lower than actual gas temperatures.

Single-cluster models underestimate true mass by a factor of 2-3, while superposition models improve accuracy.

Abstract

Several lines of evidence have suggested that the the galaxy cluster Cl~0024+17, an apparently relaxed system, is actually a collision of two clusters, the interaction occurring along our line of sight. In this paper we present a high-resolution $N$-body/hydrodynamics simulation of such a collision. We have created mock X-ray observations of our simulated system using MARX, a program that simulates the on-orbit performance of the Chandra X-ray Observatory. We analyze these simulated data to generate radial profiles of the surface brightness and temperature. At later times, $t = 2.0-3.0$ Gyr after the collision, the simulated surface brightness profiles are better fit by a superposition of two $\beta$-model profiles than a single profile, in agreement with the observations of Cl~0024+17. In general, due to projection effects, much of the post-collision density and temperature structure of the clusters is not seen in the observations. In particular, the observed temperatures from spectral fitting are much lower than the temperature of the hottest gas. We determine from our fitted profiles that if the system is modeled as a single cluster, the hydrostatic mass estimate is a factor $\sim$2-3 less than the actual mass, but if the system is modeled as two galaxy clusters in superposition, a hydrostatic mass estimation can be made which is accurate to within $\sim$10%. We examine some implications of these results for galaxy cluster X-ray surveys.