An X-ray/optical study of the geometry and dynamics of MACS   J0140.0-0555, a massive post-collision cluster merger
  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 of 875 (+70/-100) km/s (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 is small compared to typical collision velocities of several 1000 km/s. 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(<75 kpc) = (5.6+/- 0.5)*10^13 M_sun for the north-western cluster component and a much less certain estimate of (1.5-3)*10^13 M_sun for the south-eastern subcluster. These values are in good agreement with our X-ray mass estimates which yield a total mass of MACS J0140.0-0555 of M(<r_500) ~ (6.8-9.1)*10^14 M_sun. ...... 
I-Ting Ho, Harald Ebeling, and Johan Richard
astro-ph.CO