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