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Riding the Wake of a Merging Galaxy Cluster

Observations using the OASIS integral field spectrograph on the William Herschel Telescope (WHT) have revealed a long, thin plume of ionised gas stretching out from the brightest cluster galaxy (BCG) of Abell 2146 (z=0.243) (Canning et al. 2012). Extended optical emission-line nebulae are not uncommon in the cores of clusters, but the discovery of this particular structure is unexpected, as the host cluster is in the throes of a major merger event (Russell et al. 2010).
How can a >15kpc long plume survive in the environment of such a turbulent intracluster medium? Chandra X-ray observations of the system show that a merging subcluster has created large shock fronts, each several hundred kiloparsecs across. These surround a dense, relatively cool X-ray core which is being stripped of its material in the collision.

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Title: Sunyaev-Zel'dovich observation of the Bullet-like cluster A2146 with the Arcminute Microkelvin Imager
Authors: Carmen Rodriguez-Gonzalvez, Malak Olamaie, Matthew L. Davies, Andy C. Fabian, Farhan Feroz, Thomas M. O. Franzen, Keith J. B. Grainge, Michael P. Hobson, Natasha Hurley-Walker, Anthony N. Lasenby N., Guy G. Pooley, Helen R. Russell, Jeremy S. Sanders, Richard D. E. Saunders, Anna M. M. Scaife, Michel P. Schammel, Paul F. Scott, Timothy W. Shimwell, David J. Titterington, Elizabeth M. Waldram, Jonathan T. L. Zwart

We present 13.9-18.2 GHz observations of the Sunyaev-Zel'dovich (SZ) effect towards A2146 using the Arcminute Microkelvin Imager (AMI). The cluster is detected with a peak SNR ratio of 13 sigma in the radio source subtracted map. Comparison of the SZ and X-ray images suggests that they both have extended regions which lie approximately perpendicular to one another, with their emission peaks significantly displaced. These features indicate non-uniformities in the distributions of the gas temperature and pressure, indicative of a cluster merger. We use a Bayesian cluster analysis to explore the high-dimensional parameter space of the cluster-plus-sources model to obtain cluster parameter estimates in the presence of radio point sources, receiver noise and primordial CMB anisotropy; the probability of SZ + CMB primordial structure + radio sources + receiver noise to CMB + radio sources + receiver noise is 3 x 10^{6}:1. We compare the results from three different cluster models. Our preferred model exploits the observation that the gas fractions do not appear to vary greatly between clusters. Given the relative masses of the two merging systems in A2146, the mean gas temperature can be deduced from the virial theorem (assuming all of the kinetic energy is in the form of internal gas energy) without being affected significantly by the merger event, provided the primary cluster was virialized before the merger. In this model we fit a simple spherical isothermal beta-model, despite the inadequacy of this model for a merging system like A2146, and assume the cluster follows the mass-temperature relation of a virialized, singular, isothermal sphere. We note that this model avoids inferring large-scale cluster parameters internal to r_200 under the widely used assumption of hydrostatic equilibrium. We find that at r_200 M_T= 4.1 0.5 x 10^{14} h^{-1}M_sun and T=4.5 0.5 keV.

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