2012MNRAS.426.1992H

We investigate the physical properties, geometry and dynamics of the massive cluster merger MACSJ0140.0-0555 (z = 0.451) using X-ray and optical diagnostics. Featuring two galaxy overdensities separated by about 250kpc in projection on the sky, and a single peak in the X-ray surface brightness distribution located between them, MACSJ0140.0-0555 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 ( ∼ 8.5keV) with no clear signs of cool cores or shock fronts. Spectroscopic follow-up of galaxies in the field of MACSJ0140.0-0555 yields a velocity dispersion of \mn 875\mo –\mn 100\mo +\mn 70\mspace\mi km\mspace\mi s\mo –\mn 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±25km/s is small compared to typical collision velocities of several 1000km/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(<75kpc) = (5.6±0.5) x 10¹³ M_☉ for the north-western cluster component and a much less certain estimate of (1.5–3) x 10¹³ 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 MACSJ0140.0-0555 of M(<r₅₀₀) ∼ (6.8–9.1) x 10¹⁴ M_☉.

Although all optical and X-ray properties of MACSJ0140.0-0555 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 MACSJ0140.0-0555 has the potential of yielding constraints on the self-interaction cross-section of dark matter similar to those obtained for MACSJ0025.4-1222, although the smaller projected separation of the two cluster components makes the measurement more challenging.

Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.