Clash of Titans: a MUSE dynamical study of the extreme cluster merger
  SPT-CLJ0307-6225
  We present VLT/MUSE spectroscopy, along with archival Gemini/GMOS
spectroscopy, Magellan/Megacam imaging, and Chandra X-ray emission for
SPT-CLJ0305-6225, a z=0.58 galaxy cluster. A large BCG-SZ centroid separation
and a highly disturbed X-ray morphology classifies SPT-CLJ0307-6225 as a major
merging cluster. Furthermore, the galaxy density distribution shows two main
overdensities with separations of 0.144' and 0.017' to their respective BCGs.
We characterize the central regions of the two colliding structures, namely
0307-6225N and 0307-6225S. We find velocity derived masses of $M_{200,N}=$ 2.42
$\pm$ 1.40 $\times10^{14}$ M$_\odot$ and $M_{200,S}=$ 3.13 $\pm$ 1.87
$\times10^{14}$ M$_\odot$, with a line-of-sight velocity difference between the
two structures of $|\Delta v| = 342$ km s$^{-1}$. The total dynamically derived
mass is consistent with the SZ derived mass of 7.63 h$_{70}^{-1}$ $\pm$ 1.36
$\times10^{14}$ M$_\odot$. We model the merger using the Monte Carlo Merger
Analysis Code, estimating a merging angle of 36$^{+14}_{-12}$ degrees with
respect to the plane of the sky. Comparing with simulations of a merging system
with a mass ratio of 1:3, we find that the best scenario is that of an ongoing
merger that began 0.96$^{+0.31}_{-0.18}$ Gyr ago, which could be close to
turnaround. We also characterize the galaxy population using the H$\delta$ and
[OII] $\lambda 3727$ \AA \ lines. We find that most of the emission-line
galaxies belong to 0307-6225S, close to the X-ray peak position, with a third
of them corresponding to red-cluster sequence galaxies, and the rest to blue
galaxies with velocities consistent with recent periods of accretion. Moreover,
we suggest that 0307-6225S suffered a previous merger, evidenced through the
two equally bright BCGs at the center with a velocity difference of $\sim$674
km s$^{-1}$.
