Astron. Nachr. / AN 000, No. 00, 1 – 6 (0000) / DOI please set DOI! The XMM Cluster Outskirts Project (X-COP) D. Eckert1;?, S. Ettori2;3, E. Pointecouteau4;5, S. Molendi6, S. Paltani1, C. Tchernin7, and The X-COP collaboration 1 Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland 2 INAF - Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy 3 INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy 4 CNRS; IRAP; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France 5 Universite´ de Toulouse; UPS-OMP; IRAP; Toulouse, France 6 INAF - IASF-Milano, Via E. Bassini 15, 20133 Milano, Italy 7 Center for Astronomy, Institute for Theoretical Astrophysics, Heidelberg University, Philosophenweg 12, 69120 Hei- delberg, Germany The dates of receipt and acceptance should be inserted later Key words X-rays: galaxies: clusters - Galaxies: clusters: general - Galaxies: clusters: intracluster medium - cosmology: large-scale structure Galaxy clusters are thought to grow hierarchically through the continuous merging and accretion of smaller structures across cosmic time. In the Local Universe, these phenomena are still active in the outer regions of massive clusters (R > R500), where the matter distribution is expected to become clumpy and asymmetric because of the presence of accreting structures. We present the XMM-Newton Cluster Outskirts Project (X-COP), which targets the outer regions of a sample 14 of 13 massive clusters (M500 > 3 × 10 M ) in the redshift range 0.04-0.1 at uniform depth. The sample was selected based on the signal-to-noise ratio in the Planck Sunyaev-Zeldovich (SZ) survey with the aim of combining high-quality X-ray and SZ constraints throughout the entire cluster volume. Our observing strategy allows us to reach a sensitivity of 3 × 10−16 ergs cm−2 s−1 arcmin−2 in the [0.5-2.0] keV range thanks to a good control of systematic uncertainties. The combination of depth and field of view achieved in X-COP will allow us to pursue the following main goals: i) measure the distribution of entropy and thermal energy to an unprecedented level of precision; ii) assess the presence of non-thermal pressure support in cluster outskirts; iii) study the occurrence and mass distribution of infalling gas clumps. We illustrate the capabilities of the program with a pilot study on the cluster Abell 2142. c 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Sembolini et al. 2016; Voit et al. 2005). However, non- gravitational processes induce an additional injection of en- In the hierarchical structure formation paradigm, galaxy tropy and can therefore be traced through the departures clusters are expected to form through the continuous merg- from the theoretical predictions (Chaudhuri et al. 2012). ing and accretion of smaller structures (see Kravtsov & Bor- Such departures have been observed for a long time in clus- gani 2012, for a review). In the local Universe, such pro- ter cores, where gas cooling and feedback from supernovae cesses should be observable in the outer regions of massive and active galactic nuclei are important (e.g. David et al. clusters, where galaxies and galaxy groups are infalling for 1996; Ponman et al. 1999; Pratt et al. 2010). More recently, the first time and smooth material is continuously accreted several works reported a deficit of entropy in massive clus- ters around the virial radius (see Reiprich et al. 2013, for a arXiv:1611.05051v1 [astro-ph.CO] 15 Nov 2016 from the surrounding cosmic web. review), which has been interpreted as a lack of thermaliza- The hot plasma in galaxy clusters is expected to be 7 8 tion of the ICM induced, e.g., by an incomplete virializa- heated to high temperatures (10 − 10 K) through shocks tion of the gas (e.g. Bonamente et al. 2013; Ichikawa et al. and adiabatic compression at the boundary between the 2013; Kawaharada et al. 2010), non-equilibration between free-falling gas and the virialized intra-cluster medium electrons and ions (Hoshino et al. 2010), non-equilibrium (ICM, Tozzi et al. 2000). The thermodynamical properties ionization (e.g. Fujita et al. 2008), or weakening of the ac- of the gas retain information on the processes leading to cretion shocks (Lapi et al. 2010). However, these models the thermalization of the gas in the cluster’s potential well, −2=3 have received little support from cosmological simulations which is encoded in the gas entropy K = kT ne . Gravi- so far (e.g. Avestruz et al. 2015; Lau et al. 2015; Vazza et al. tational collapse models predict that the entropy of stratified 2010). cluster atmospheres increases steadily with radius, follow- ing a power law with index ∼ 1:1 (Borgani et al. 2005; The gas content of infalling dark-matter halos interacts with the ICM and is stripped from its parent halo through ? Corresponding author: e-mail: [email protected] the influence of the ram pressure applied by the ICM of the c 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2 D. Eckert et al.: Instructions for authors main cluster. This process is expected to be the main mech- 2 Sample selection anism through which the infalling gas is heated up and viri- alized into the main dark-matter halo (Gunn & Gott 1972; Heinz et al. 2003; Roediger et al. 2015; Vollmer et al. 2001) To implement the strategy presented above, we selected a and it is believed to be key to the evolution of the cluster list of the most suitable targets to conduct our study. The galaxy population by quenching rapidly the star formation criteria used for the selection are the following: activity in clusters (Bahe´ & McCarthy 2015; Roediger & Bruggen¨ 2008). Recent observational evidence suggest that 1. SNR > 12 in the PSZ1 catalog (Planck Collabora- thermal conduction in the ICM is strongly inhibited (e.g. tion XXIX 2014): This condition is necessary to target Gaspari & Churazov 2013; Sanders et al. 2013). The long the most significant Planck detections and ensure that conduction timescale therefore delays the virialization of the SZ effect from all clusters be detected beyond R ; the stripped, low-entropy gas inside the potential well of the 500 2. Apparent size θ500 > 10 arcmin: Given the limited main cluster (De Grandi et al. 2016; Eckert et al. 2014), angular resolution of our reconstructed SZ maps (∼ 7 which causes the ICM in the outer regions of massive clus- arcmin), this condition ensures that all the clusters are ters to be clumpy (Mathiesen et al. 1999; Nagai & Lau 2011; well-resolved, such that the contamination of SZ flux Vazza et al. 2013). Since the X-ray emissivity depends on from the core is low beyond R ; the squared gas density, inhomogeneities in the gas distri- 500 3. Redshift in the range 0:04 < z < 0:1: This criterion bution lead to an overestimation of the mean gas density allows us to cover most of the azimuth out to R with (Eckert et al. 2015b; Nagai & Lau 2011; Simionescu et al. 200 5 XMM-Newton pointings (one central and four offset) 2011), which biases the measured entropy low. This effect whilst remaining resolved by Planck; needs to be taken into account when measuring the entropy 21 −2 4. Galactic NH < 10 cm : Since we are aiming at associated with the bulk of the ICM. In addition, large-scale maximizing the sensitivity in the soft band, this con- accretion patterns in the direction of the filaments of the dition makes sure that the soft X-ray signal is weakly cosmic web induce asymmetries in the gas distribution (e.g. absorbed. Eckert et al. 2012; Roncarelli et al. 2013; Vazza et al. 2011). Such filaments are expected to host the densest and hottest phase of the warm-hot intergalactic medium (e.g. Cen & This selection yields a set of the 15 most suitable targets Ostriker 1999; Dave´ et al. 2001; Eckert et al. 2015a), which for our goals. We excluded three clusters (A2256, A754, and are expected to account for most of the missing baryons in A3667) because of very complicated morphologies induced the local Universe. by violent merging events, which might hamper the analy- sis of the Planck data given the broad Planck beam. The re- maining 12 clusters selected for our study are listed in Table In this paper, we present the XMM-Newton cluster out- 1, together with their main properties. A uniform 25 ks map- skirts project (X-COP), a very large programme on XMM- ping with XMM-Newton was performed for 10 of these sys- Newton that aims at advancing significantly our knowledge tems in the framework of the X-COP very large programme of the physical conditions in the outer regions of galaxy (Proposal ID 074441, PI: Eckert), which was approved dur- 1 clusters (R > R500 ). X-COP targets a sample of 13 mas- ing XMM-Newton AO-13 for a total observing time of 1.2 sive, nearby clusters selected on the basis of their high Ms. The remaining 2 systems (A3266 and A2142) were signal-to-noise ratio (SNR) in the Planck all-sky survey mapped by XMM-Newton previously. Although the avail- of Sunyaev-Zeldovich (SZ, Sunyaev & Zeldovich 1972) able observations of A3266 do not extend all the way out to sources (Planck Collaboration XXIX 2014; Planck Col- R200, they are still sufficient for some of our objectives and laboration XXVII 2015). In the recent years, the progress we include them in the present sample. Finally, we add Hy- achieved in the sensitivity of SZ instruments allowed to dra A/A780 to the final sample. While the SZ signal from extend the measurements of the pressure profile of galaxy this less massive cluster is not strong enough to be detected clusters out to the virial radius and beyond (Planck Collab- beyond R500, a deep, uniform XMM-Newton mapping exists oration V 2013; Sayers et al.