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MC^2: A deeper look at ZwCl 2341.1+0000 with Bayesian galaxy clustering and weak lensing analyses

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 Added by Bryant Benson
 Publication date 2017
  fields Physics
and research's language is English




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ZwCl 2341.1+0000, a merging galaxy cluster with disturbed X-ray morphology and widely separated ($sim$3 Mpc) double radio relics, was thought to be an extremely massive ($10-30 times 10^{14} M_odot$) and complex system with little known about its merger history. We present JVLA 2-4 GHz observations of the cluster, along with new spectroscopy from our Keck/DEIMOS survey, and apply Gaussian Mixture Modeling to the three-dimensional distribution of 227 confirmed cluster galaxies. After adopting the Bayesian Information Criterion to avoid overfitting, which we discover can bias total dynamical mass estimates high, we find that a three-substructure model with a total dynamical mass estimate of $9.39 pm 0.81 times 10^{14} M_odot$ is favored. We also present deep Subaru imaging and perform the first weak lensing analysis on this system, obtaining a weak lensing mass estimate of $5.57 pm 2.47 times 10^{14} M_odot$. This is a more robust estimate because it does not depend on the dynamical state of the system, which is disturbed due to the merger. Our results indicate that ZwCl 2341.1+0000 is a multiple merger system comprised of at least three substructures, with the main merger that produced the radio relics occurring near to the plane of the sky, and a younger merger in the North occurring closer to the line of sight. Dynamical modeling of the main merger reproduces observed quantities (relic positions and polarizations, subcluster separation and radial velocity difference), if the merger axis angle of $sim$10$^{+34}_{-6}$ degrees and the collision speed at pericenter is $sim$1900$^{+300}_{-200}$ km/s.



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The galaxy cluster ZwCl 2341.1+0000 is a merging system at z=0.27, which hosts two radio relics and a central, faint, filamentary radio structure. The two radio relics have unusually flat integrated spectral indices of -0.49 +/- 0.18 and -0.76 +/- 0.17, values that cannot be easily reconciled with the theory of standard diffusive shock acceleration of thermal particles at weak merger shocks. We present imaging results from XMM-Newton and Chandra observations of the cluster, aimed to detect and characterise density discontinuities in the ICM. As expected, we detect a density discontinuity near each of the radio relics. However, if these discontinuities are the shock fronts that fuelled the radio emission, then their Mach numbers are surprisingly low, both <=2. We studied the aperture of the density discontinuities, and found that while the NW discontinuity spans the whole length of the NW radio relic, the arc spanned by the SE discontinuity is shorter than the arc spanned by the SE relic. This startling result is in apparent contradiction with our current understanding of the origin of radio relics. Deeper X-ray data are required to confirm our results and to determine the nature of the density discontinuities.
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