Revising the merger scenario of the galaxy cluster Abell 1644: a new gas
  poor structure discovered by weak gravitational lensing
  The galaxy cluster Abell 1644 ($\bar{z}=0.047$) is known for its remarkable
spiral-like X-ray emission. It was previously identified as a bimodal system,
comprising the subclusters, A1644S and A1644N, each one centered on a giant
elliptical galaxy. In this work, we present a comprehensive study of this
system, including new weak-lensing and dynamical data and analysis plus a
tailor-made hydrodynamical simulation. The lensing and galaxy density maps
showed a structure in the North that could not be seen on the X-ray images. We,
therefore, rename the previously known northern halo as A1644N1 and the new one
as A1644N2. Our lensing data suggests that those have fairly similar masses:
$M_{200}^{\rm N1}=0.90_{-0.85}^{+0.45} \times10^{14}$ and $M_{200}^{\rm
N2}=0.76_{-0.75}^{+0.37} \times10^{14}$ M$_\odot$, whereas the southern
structure is the main one: $M_{200}^{\rm S}=1.90_{-1.28}^{+0.89}\times 10^{14}$
M$_\odot$. Based on the simulations, fed by the observational data, we propose
a scenario where the remarkable X-ray characteristics in the system are the
result of a collision between A1644S and A1644N2 that happened $\sim$1.6 Gyr
ago. Currently, those systems should be heading to a new encounter, after
reaching their maximum separation.
