The Pre-merger Impact Velocity of the Binary Cluster A1750 from X-ray, Lensing and Hydrodynamical Simulations

TL;DR

This study combines X-ray observations and hydrodynamical simulations to estimate the pre-merger impact velocity of galaxy clusters, revealing velocities higher than typical LCDM predictions but still within allowed ranges, aiding dark matter research.

Contribution

It provides a novel method to determine pre-merger velocities using combined observational and simulation data, focusing on the A1750 cluster system.

Findings

Deprojected initial impact velocity of 1460 km/sec for A1750

Simulations accurately reproduce X-ray temperature and redshift data

Pre-merger velocities can test dark matter models

Abstract

Since the discovery of the "bullet cluster" several similar cases have been uncovered suggesting relative velocities well beyond the tail of high speed collisions predicted by the concordance LCDM model. However, quantifying such post-merger events with hydrodynamical models requires a wide coverage of possible initial conditions. Here we show that it is simpler to interpret pre-merger cases, such as A1750, where the gas between the colliding clusters is modestly affected, so that the initial conditions are clear. We analyze publicly available Chandra data confirming a significant increase in the projected X-ray temperature between the two cluster centers in A1750 consistent with our expectations for a merging cluster. We model this system with a self-consistent hydrodynamical simulation of dark matter and gas using the FLASH code. Our simulations reproduce well the X-ray data, and the measured redshift difference between the two clusters in the phase before the first core passage viewed at an intermediate projection angle. The deprojected initial relative velocity derived using our model is 1460 km/sec which is considerably higher than the predicted mean impact velocity for simulated massive haloes derived by recent LCDM cosmological simulations, but it is within the allowed range. Our simulations demonstrate that such systems can be identified using a multi-wavelength approach and numerical simulations, for which the statistical distribution of relative impact velocities may provide a definitive examination of a broad range of dark matter scenarios.