Planck intermediate results. VIII. Filaments between interacting clusters

Planck intermediate results. VIII. Filaments between interacting clusters

Planck intermediate results
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Context:About half of the baryons of the Universe are expected to be in the form of filaments of hot and low-density intergalactic medium. Most of these baryons remain undetected even by the most advanced X-ray observatories, which are limited in sensitivity to the diffuse low-density medium.

Aims:The Planck satellite has provided hundreds of detections of the hot gas in clusters of galaxies via the thermal Sunyaev-Zel’dovich (tSZ) effect and is an ideal instrument for studying extended low-density media through the tSZ effect. In this paper we use the Planck data to search for signatures of a fraction of these missing baryons between pairs of galaxy clusters.

Methods:Cluster pairs are good candidates for searching for the hotter and denser phase of the intergalactic medium (which is more easily observed through the SZ effect). Using an X-ray catalogue of clusters and the Planck data, we selected physical pairs of clusters as candidates. Using the Planck data, we constructed a local map of the tSZ effect centred on each pair of galaxy clusters. ROSAT data were used to construct X-ray maps of these pairs. After hmodelling and subtracting the tSZ effect and X-ray emission for each cluster in the pair, we studied the residuals on both the SZ and X-ray maps.

Results:For the merging cluster pair A399-A401 we observe a significant tSZ effect signal in the intercluster region beyond the virial radii of the clusters. A joint X-ray SZ analysis allows us to constrain the temperature and density of this intercluster medium. We obtain a temperature of   keV (consistent with previous estimates) and a baryon density of   cm.

Conclusions:The Planck satellite mission has provided the first SZ detection of the hot and diffuse intercluster gas.

1 Introduction

A sizeable fraction of the baryons of the Universe are expected to be in the form of the WHIM (warm-hot intergalactic medium) and remain undetected at low redshifts. The WHIM is expected to exist mostly in filaments but also around and between massive clusters. These missing baryons are supposed to be in a low-density, low-temperature phase (overdensities between 5 to 200 times the critical density and   K, Cen99), making the amount of X-rays produced by the WHIM too small to be detected with current X-ray facilities. By contrast, their detection could be possible via the Sunyaev-Zel’dovich (SZ hereafter) effect (SZ) produced by the inverse Compton scattering between the cosmic microwave background (CMB) photons and the electrons of the WHIM. As the SZ effect is proportional to the electron pressure in the medium, low-density and low-temperature regions can be detected provided their integrated signal is strong enough. Planck’s relatively poor resolution becomes an advantage in this situation since it permits scanning of wide regions of the sky that can later be integrated to increase the signal-to-noise ratio of the diffuse (but intrinsically large-scale) SZ signal.

The full-sky coverage and wide frequency range of the Planck satellite mission makes it possible to produce reliable maps of the tSZ emission (2011A&A...536A...8P; 2011A&A...536A...9P; 2011A&A...536A..10P). In particular, Planck is better suited than ground experiments to detecting diffuse SZ signals, such as the WHIM, which can extend over relatively large angular scales. Ground experiments can be affected at large angular scales by atmospheric fluctuations that need to be removed. This removal process can distort the modes that include the large angular scale signals. Planck data do not suffer from these limitations and can use their relatively poor angular resolution (when compared to some ground experiments) to its advantage. Indeed, diffuse low surface brightness objects can be resolved and detected by Planck. Finally, the wide frequency coverage and extremely high sensitivity of Planck allows for detailed foreground (and CMB) removal that otherwise would overwhelm the weak signal of the WHIM.

The gas around clusters is expected to be hotter and denser than the WHIM in filaments, making direct detection of the cluster gas more likely. In addition, the increase of pressure caused by the merging process enhances the SZ signal, making it easier to detect the gas between pairs of interacting clusters. In the process of hierarchical formation clusters assemble via continuous accretion and merger events. Therefore, the bridge of intercluster matter between them is expected to be of higher density, temperature, and thus thermal pressure than the average WHIM matter found in cosmic filaments (dolag2006).

The Planck satellite (planckmission) has the potential to detect these filamentary structures directly via the SZ effect. Suitable targets for Planck are close objects that subtend large solid angles and therefore have high integrated SZ fluxes. Alternatively, regions between mergers (filaments between pairs of clusters) or extremely deep gravitational wells (superclusters such as the Shapley or Corona Borealis, cacho2009) will contain diffuse gas with increased pressure that could be detected by Planck. For this work, we concentrate on searching for diffuse filamentary-like structure between pairs of merging clusters. We used the MCXC (Meta-Catalog of X-Ray Detected Clusters) catalogue of clusters of galaxies (Piffaretti2011) and the Planck data to select a sample of pairs of merging clusters to study the properties of the gas in the intercluster region.

Indirect WHIM detections have been claimed through absorption lines in the X-ray (and UV) band (Richter08). There is also evidence of filamentary structure in the intercluster region from X-ray observations of several well-known merging cluster pairs such as A222-A223 (Werner2008), A399-A401 (Sakelliou2004), A3391-A3395 (Tittley2001), and from the double cluster A1758 (Durret). The pairs of clusters A3391-A3395 (separated by about 50 on the sky and at redshifts z=0.051 and z=0.057, respectively (Tittley2001)) and more specially, A399-A401 (separated by about 40 on the sky and at redshifts z=0.0724 and z=0.0737, respectively) are of particular interest for the purpose of this paper, given their geometry and angular separation. This is sufficient to allow Planck to resolve the individual cluster components.

For A399-A401, earlier observations show an excess of X-ray emission above the background level in the intercluster region. Using XMM data, Sakelliou2004 obtained best-fitting models in the intercluster region that indicated such an excess. Both clusters are classified as non-cool-core clusters and show weak radio halos (Murgia2010). These two facts could be an indication of a past interaction between the two clusters. Fujita1996 analysed ASCA data of the intercluster region and found a relatively high temperature in this region. They suggested a pre-merger scenario but did not rule out a past interaction. Fabian1997 used HRI ROSAT data and found a prominent linear feature in A399 pointing towards A401. They suggested that this could be evidence of a past interaction. Using Suzaku observations, Fujita2008 found that the intercluster region has a relatively high metallicity of 0.2 solar. These works estimated that the filamentary bridge has an electron density of   cm (Fujita1996; Sakelliou2004; Fujita2008).

In this paper we concentrate on pairs of merging clusters including A399-A401 and we study the physical properties of the gas in the intercluster region via a combined analysis of the tSZ effect and the X-ray emission. The paper is organized as follows. Section 2 gives a brief description of the Planck data used for this study. In Section LABEL:selection we describe the selection procedure used to identify the most suitable pairs of clusters for the analysis. We search for pairs of clusters for which the contribution of the SZ effect to the signal is significant in the intercluster medium. Section LABEL:xrays describes the X-ray ROSAT observations for the selected pairs of clusters. In section LABEL:Models we model the SZ and X-ray emission from the clusters assuming spherical symmetry and subtract them from the data. In section LABEL:analysis the SZ and X-ray residuals are fitted to a simplified filament model to characterize the physical properties of the intercluster region. Section LABEL:discussions discusses our main results focusing on the limitations imposed by the cluster spherical symmetry assumption and alternative non-symmetric scenarios. Finally, we conclude in Section LABEL:conclusions.

2 Planck data

Table 1: Main physical parameters of the selected pairs of clusters. Values quoted with NA are unknown.


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