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The XMM Cluster Outskirts Project (X-COP): Physical Conditions of Abell 2142 up to the Virial Radius C

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The XMM Cluster Outskirts Project (X-COP): Physical Conditions of Abell 2142 up to the Virial Radius C A&A 595, A42 (2016) Astronomy DOI: 10.1051/0004-6361/201628183 & c ESO 2016 Astrophysics The XMM Cluster Outskirts Project (X-COP): Physical conditions of Abell 2142 up to the virial radius C. Tchernin1; 2, D. Eckert2; 3, S. Ettori4; 5, E. Pointecouteau6; 11, S. Paltani2, S. Molendi3, G. Hurier7; 12, F. Gastaldello3; 8, E. T. Lau9, D. Nagai9, M. Roncarelli4, and M. Rossetti10; 3 1 Center for Astronomy, Institute for Theoretical Astrophysics, Heidelberg University, Philosophenweg 12, 69120 Heidelberg, Germany e-mail: [email protected] 2 Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland 3 INAF–IASF-Milano, via E. Bassini 15, 20133 Milano, Italy 4 INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy 5 INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy 6 CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France 7 Centro de Estudios de Fisica del Cosmos de Aragon, Plaza San Juan 1, Planta-2, 44001 Teruel, Spain 8 Department of Physics and Astronomy, University of California at Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697-4575, USA 9 Department of Physics, Yale University, New Haven, CT 06520, USA 10 Università degli studi di Milano, Dip. di Fisica, via Celoria 16, 20133 Milano, Italy 11 Université de Toulouse, UPS-OMP, IRAP, 38000 Toulouse, France 12 Institut d’Astrophysique Spatiale, CNRS, UMR8617, Université Paris-Sud 11, Batiment 121, Orsay, France Received 25 January 2016 / Accepted 17 June 2016 ABSTRACT Context. Galaxy clusters are continuously growing through the accretion of matter in their outskirts. This process induces inho- mogeneities in the gas density distribution (clumping) that need to be taken into account to recover the physical properties of the intracluster medium (ICM) at large radii. Aims. We studied the thermodynamic properties in the outskirts (R > R500) of the massive galaxy cluster Abell 2142 by combining the Sunyaev Zel’dovich (SZ) effect with the X-ray signal. Methods. We combined the SZ pressure profile measured by Planck with the XMM-Newton gas density profile to recover radial profiles of temperature, entropy, and hydrostatic mass out to 2 × R500. We used a method that is insensitive to clumping to recover the gas density, and we compared the results with traditional X-ray measurement techniques. Results. When taking clumping into account, our joint X-SZ entropy profile is consistent with the predictions from pure gravitational collapse, whereas a significant entropy flattening is found when the effect of clumping is neglected. The hydrostatic mass profile recovered using joint X-SZ data agrees with that obtained from spectroscopic X-ray measurements and with mass reconstructions obtained through weak lensing and galaxy kinematics. Conclusions. We found that clumping can explain the entropy flattening observed by Suzaku in the outskirts of several clusters. When using a method that is insensitive to clumping for the reconstruction of the gas density, the thermodynamic properties of Abell 2142 are compatible with the assumption that the thermal gas pressure sustains gravity and that the entropy is injected at accretion shocks, with no need to evoke more exotic physics. Our results highlight the need for X-ray observations with sufficient spatial resolution, and large collecting area, to understand the processes at work in cluster outer regions. Key words. X-rays: galaxies: clusters – radiation mechanisms: thermal – galaxies: clusters: intracluster medium – cosmology: observations – submillimeter: general 1. Introduction the form of turbulence, bulk motions, cosmic rays, and magnetic fields is expected to be significant, even if still poorly constrained Galaxy clusters are the largest gravitationally bound systems by theory and simulations. in the Universe. According to the concordance cosmological Spectroscopic X-ray measurements of cluster outskirts be- model, they are the latest structures to be formed. For this came possible recently with the Suzaku experiment thanks to reason, they are expected to continue growing at the present its low particle background (Mitsuda et al. 2007). With the epoch through the accretion of matter in their outskirts. Thus, help of Suzaku, many bright galaxy clusters have been ob- information on the processes governing structure formation can served out to the viral radius (∼R200; e.g., Reiprich et al. 2009; be obtained through the study of galaxy cluster outskirts (see Hoshino et al. 2010; Akamatsu et al. 2011; Simionescu et al. Reiprich et al. 2013, for a review). The distribution of matter 2011; Walker et al. 2012, 2013; Urban et al. 2014; Okabe et al. in the outer regions of galaxy clusters is expected to become 2014). These works studied the radial profiles of the density, clumpy (Nagai & Lau 2011; Vazza et al. 2013) and asymmet- temperature, and entropy out to R200. In several cases, the au- ric (Vazza et al. 2011), and the impact of nonthermal energy in thors observed a flattening of the entropy profile beyond R500 Article published by EDP Sciences A42, page 1 of 22 A&A 595, A42 (2016) compared to the expectation of the self-similar accretion model In this paper, we combine SZ and X-ray observations (Voit et al. 2005). Several studies also observed a decrease in the (X-SZ) from the Planck and XMM-Newton satellites to study hydrostatic mass profile in the same radial range, which might the outskirts of Abell 2142. This cluster belongs to the sam- suggest that the medium is out of hydrostatic equilibrium. This ple selected in the framework of the XMM Cluster Outskirts could be caused by a significant nonthermal pressure in the form Project (X-COP), a very large program on XMM-Newton which of turbulence, bulk motions, or cosmic rays (e.g., Vazza et al. aims at studying the outskirts of an SZ-selected sample of 2009; Lau et al. 2009; Battaglia et al. 2013), nonequilibration 13 massive, nearby clusters. Abell 2142 is a massive cluster 15 between electrons and ions (Hoshino et al. 2010; Avestruz et al. (M200 ∼ 1:3 × 10 M ; Munari et al. 2014) at a redshift of 2015), or weakening of the accretion shocks (Lapi et al. 2010; 0.09 (Owers et al. 2011). The cluster hosts a moderate cool core 2 Fusco-Femiano & Lapi 2014). (K0 = 68 keV cm ; Cavagnolo et al. 2009) and exhibits multi- ple concentric cold fronts in its central regions (Markevitch et al. Alternatively, Simionescu et al.(2011) proposed that the 2000; Rossetti et al. 2013), which are indicative of ongoing measured gas density is overestimated because of gas clump- sloshing activity extending out to 1 Mpc from the cluster cen- ing, which would lead to an underestimated entropy (see ter (Rossetti et al. 2013). The sloshing activity may have trig- also Eckert et al. 2013b; Walker et al. 2013; Urban et al. 2014; gered the formation of an unusual radio halo (Farnsworth et al. Morandi et al. 2013; Morandi & Cui 2014). Recently, several 2013). Owers et al.(2011) studied the three-dimensional (3D) numerical studies have focused on quantifying the effect of gas galaxy distribution out to ∼2 Mpc from the cluster core and inhomogeneities on X-ray observations (see, e.g., Nagai & Lau identified several substructures associated with minor mergers. 2011; Zhuravleva et al. 2013; Eckert et al. 2015; Vazza et al. Eckert et al.(2014) discovered an infalling galaxy group located 2013; Roncarelli et al. 2013). For instance, using hydrodynami- ∼1:5 Mpc northeast (NE) of the main cluster. This subcluster is cal simulations Zhuravleva et al.(2013) showed that the density in the process of being stripped from its hot gas by the ram pres- distribution inside a shell at a given distance from the cluster sure applied by the main cluster, forming a spectacular X-ray center can be described by a lognormal distribution plus a high- tail. On larger scales, A 2142 is located in the core of a collaps- density tail (see also Rasia et al. 2014; Khedekar et al. 2013). ing supercluster (Einasto et al. 2015; Gramann et al. 2015). To- While the lognormal distribution contains information about the gether, these studies reveal that A 2142 is a dynamically active bulk of the intracluster medium (ICM), the high-density tail is cluster located at a node of the cosmic web. due to the presence of infalling gas clumps. The authors showed The paper is organized as follows. In Sect.2 we describe the that the median of the distribution coincides with the mode of analysis of the X-ray data to obtain radial profiles of the surface- the lognormal distribution, whereas the mean is biased high by brightness, temperature, and metal abundance of A 2142. In the presence of clumps. This result has been confirmed obser- Sect.3, we perform a deprojection of the profiles of X-ray sur- vationally by Eckert et al.(2015), where the authors reproduced face brightness and SZ y parameter from Planck assuming spher- this result using ROSAT and XMM-Newton data. These authors ical symmetry to recover the 3D gas density and pressure pro- computed the surface-brightness distribution in an annulus at files. In Sect.4, we combine the resulting SZ pressure profile ∼ × 1:2 R500 from the cluster center and showed that the median with the X-ray density profile to obtain radial profiles of en- of the distribution corresponds to the mode of the lognormal tropy, temperature, hydrostatic mass, and gas fraction. We also distribution, while the mean is shifted toward higher surface- estimate the effects of gas clumping by comparing the results ob- brightness values. They concluded that the azimuthal median tained with the azimuthal median method with those obtained us- method allows us to recover the true gas density profile even ing the traditional approach.
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