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We investigate the physical properties, geometry and dynamics of the massive cluster merger MACS J0140.0−0555 ( z  = 0.451) using X‐ray and optical diagnostics. Featuring two galaxy overdensities separated by about 250 kpc in projection on the sky, and a single peak in the X‐ray surface brightness distribution located between them, MACS J0140.0−0555 shows the tell‐tale X‐ray/optical morphology of a binary, post‐collision merger. Our spectral analysis of the X‐ray emission, as measured by our  Chandra  ACIS‐I observation of the system, finds the intra‐cluster medium to be close to isothermal (∼8.5 keV) with no clear signs of cool cores or shock fronts. Spectroscopic follow‐up of galaxies in the field of MACS J0140.0−0555 yields a velocity dispersion of875   −  100    +  70kms    −  1( n   z   = 66) and no significant evidence of bimodality or substructure along the line of sight. In addition, the difference in radial velocity between the brightest cluster galaxies of the two sub‐clusters of 144 ± 25 km s −1  is small compared to typical collision velocities of several 1000 km s −1 . A strongly lensed background galaxy at  z  = 0.873 (which features variable X‐ray emission from an active nucleus) provides the main constraint on the mass distribution of the system. We measure  M (&lt;75 kpc) = (5.6 ± 0.5) × 10 13  M ⊙  for the north‐western cluster component and a much less certain estimate of (1.5–3) × 10 13  M ⊙  for the south‐eastern sub‐cluster. These values are in good agreement with our X‐ray mass estimates which yield a total mass of MACS J0140.0−0555 of  M (&lt; r  500 ) ∼ (6.8–9.1) × 10 14  M ⊙ .  Although all optical and X‐ray properties of MACS J0140.0−0555 are consistent with a well‐advanced head‐on merger proceeding along an axis close to the plane of the sky, the degeneracy between Hubble flow and peculiar velocity prevents us from obtaining a quantitative constraint on the inclination angle of the merger axis. The lack of pronounced substructure in the cluster gas distribution and the proximity of the two optical cluster cores both in projection and in radial velocity suggest that the merger is observed well after the primary collision and, possibly, after turnaround. A weak‐lensing analysis of the mass distribution in MACS J0140.0−0555 has the potential of yielding constraints on the self‐interaction cross‐section of dark matter similar to those obtained for MACS J0025.4−1222, although the smaller projected separation of the two cluster components makes the measurement more challenging.

An X‐ray/optical study of the geometry and dynamics of MACS J0140.0−0555, a massive post‐collision cluster merger