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Issue
A&A
Volume 592, August 2016
The XXL Survey: First results
Article Number A6
Number of page(s) 5
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201527142
Published online 15 June 2016

© ESO, 2016

1. Introduction

Clusters of galaxies are promising tools that can be used to test cosmology and the predictions of General Relativity since they probe both the geometry of the universe and the growth of structure. The XXL project (Pierre et al. 2016, hereafter Paper I) is a large XMM survey at medium X-ray depth. It comprises two regions of 25 deg2 each located on the celestial equator (XMM-LSS field) and on the southern hemisphere (BCS field). The main goal of XXL is to detect and use approximately 500 galaxy clusters (0 <z< 1) to constrain the time evolution of the Dark Energy equation of state (Pierre et al. 2011).

Moreover, XXL provides an unprecedented volume between 0.5 <z< 1 with which to study the nature and evolutionary properties of groups, clusters, and superclusters of galaxies. The formation of a web of galaxies and systems of galaxies is predicted in the current cosmological paradigm where galaxies and galaxy systems form because of the constant amplification of initially very small fluctuations in the matter density. Density perturbations on scales ranging from 100 h-1 Mpc down to 10 h-1 Mpc give rise to the largest systems of galaxies, the super-clusters, ranging from rich, large super-clusters containing many massive clusters extending over 10−20 Mpc down to less massive structures containing groups and poor clusters of the order of 1013 − 1014 M⊙ each (e.g. Einasto et al. 2011, and references therein).

The superclusters, already decoupled from the Hubble flow, are not yet virialised, but most of them will collapse under the effect of gravity. At larger scales dynamical evolution proceeds at a slower rate and super-clusters have retained the memory of the initial conditions of their formation. Therefore they are important sites where we can directly witness the process of structure formation and evolution and the mass assembly to form clusters.

In this paper we analyse a supercluster of galaxies, XLSSC-e, at redshift z ~ 0.43, the highest redshift supercluster found in XXL. It was obtained as a result of a percolation analysis with a linking length of 35 Mpc applied to the sample of the 100 brightest clusters (hereafter XXL-100-GC1) detected in the XXL Survey (Pacaud et al. 2016, hereafter Paper II).

It is composed of six cluster-sized galaxy concentrations (the Abell radius, RAbell, is ~1.2 Mpc at the mean supercluster redshift). They have all been independently well detected as significantly extended X-ray sources; all of them belong to the class of C1 clusters, i.e. the most secure and uncontaminated detections in the XXL cluster sample (see Paper II); and three (XLSSC 083, XLSSC 084, XLSSC 085) are part of the 100 brightest XXL clusters. Below we describe the existing multiwavelength observations, the results obtained so far, and our conclusions. We adopted a cosmology where Ω0 = 0.282, ΩΛ = 0.718, H0 = 69.7 km s-1/Mpc, i.e. (WMAP9+BAO), plus constraints on H0 from Cepheids and type Ia supernovae, same as in Paper II.

2. Observations and data reduction

Based on a cluster search using photometric redshifts in the CFHTLS wide fields, Durret et al. (2011) identified one potential cluster at zphot = 0.48 located at RA = 32.7603, Dec = − 6.1936, and ~6.55′ away from our XLSSC 085 cluster; this corresponds approximately to the position of the BCG of XLSSC 084.

From the XXL XMM observations, we have inferred, to date, the presence of five superclusters (Paper II). With a redshift of ~0.43, XLSSC-e – which is the subject of the present paper – is the most distant one. It consists of six X-ray emitting clusters arranged in a compact structure (~ 15′ × 30′), all components residing in a single XMM pointing.

Subsequent optical spectroscopy with the WHT (Koulouridis et al. 2016, hereafter Paper XII) confirmed the redshift of the structure.

2.1. X-ray observations

The data processing and the sample selection are fully described in Paper II. The spectral analysis performed to obtain temperature and luminosity measurements and the estimate of the mass are described in Giles et al. (2016, hereafter Paper III) and Lieu et al. (2016, hereafter Paper IV). These steps are briefly summarised here. The XXL observation was filtered by soft proton flares and images, exposure maps, and detector masks were generated and processed using the Xamin pipeline (Pacaud et al. 2006). Source detection and source extent were determined through a SExtractor run followed by a dedicated XMM maximum likelihood fitting procedure. To account for the background in the spectral analysis, local backgrounds taken at the same off-axis position as the cluster were used. Cluster spectra were extracted for each of the XMM cameras and fits performed in the 0.4−7.0 keV band with an absorbed APEC (Smith et al. 2001) model modified by Galactic absorption (Kalberla et al. 2005) and with a fixed metal abundance of 0.3 Z⊙ (Anders et al. 1989). The statistic cstar was used and both source and background spectra were binned to 5 counts per bin at least (Willis et al. 2005). The typical X-ray spectrum is shown in Fig. 1.

thumbnail Fig. 1

X-ray spectra of the cluster XLSSC 082 taken from a 300 kpc aperture. The best fitting model is also shown. Data from MOS1, MOS2, and pn are plotted in black, red, and green, respectively.

The temperature (T300 kpc) and luminosity (in the 0.5−2 keV band, L300kpcXXL\hbox{$L_{\rm{300~kpc}}^{\rm{XXL}}$}) were derived within 300 kpc for each cluster as this radius is the largest radius for which a temperature could be derived for XXL-100-GC. The same aperture and procedure adopted for XXL-100-GC was used for the three clusters of XLSSC-e which are not in XXL-100-GC. As in Paper III, we adopted the MWL − T relation derived in Paper IV to obtain the mass within an overdensity of 500 (M500,MT, M500 hereafter). Individual gas masses for each cluster were obtained following the method described in Eckert et al. (2016, hereafter Paper XIII). Namely, surface-brightness profiles were extracted from the X-ray peak after correcting for vignetting and subtracting the background. To obtain the gas mass, the profiles were deprojected assuming spherical symmetry, converted into density, and integrated over the volume. These quantities are reported for each cluster in Table 1.

thumbnail Fig. 2

CFHTLS MegaCam mosaic image in the i band with the X-ray contours superimposed in green; identified X-ray clusters are encircled in magenta circles, which have a radius r = r500, and their XXL IDs indicated. The blue circles show the positions of the BCG in each cluster and the black crosses highlight the point sources excluded from the X-ray analysis. Contour levels are in logarithmic scale and they range from 4.5 counts/s/deg2 to 30 counts/s/deg2. North is up and east is on the left.

Table 1

Properties of XLSSC-e supercluster.

2.2. Optical observations

2.2.1. Photometry

The brightest cluster galaxy (BCG) for each cluster was identified by choosing the brightest galaxy in the MegaCam z filter within r ≤ 0.5 × r500 of the X-ray emission centroid (see Lavoie et al., in prep., for the full catalog of BCGs in the full sample of the 100 brightest XXL clusters).

We corrected the g and r magnitudes for extinction following Schlegel et al. (1998) and we used k corrections from Chilingarian et al. (2010)2. Assuming an absolute solar magnitude of 4.67 in r, and neglecting correction for passive evolution, we calculated the r luminosity for each BCG; finally, we calculated their mass using the relation log (M∗/L⊙) = − 0.306 + 1.097 × (g − r) from Bell et al. (2003).

2.2.2. Spectroscopy

We observed the super-cluster with the 4.2 m William Herschel Telescope (WHT) during four nights in 2013 (29−30 October and 9−10 November) using the AutoFib2+WYFFOS (AF2) spectrograph with a fibre diameter of 1.6″, covering the spectral range from 3800 Å to 7000 Å, an instrumental resolution of 4.4 Å. We limited ourselves to the central 20′ to minimise the effects of vignetting. Exposure times were 2.5 h and 3.5 h for the bright (19 ≤ rSDSS ≤ 20.5) and faint targets (20.5 ≤ rSDSS ≤ 21), respectively. During this run fibres were allocated on the BCG galaxy of each structure, and the surrounding galaxies within 1 Mpc of each BCG; a total of 15 galaxies were confirmed spectroscopically as cluster members. Further details about the data reduction and analysis can be found in Paper XII.

Table 2

Measured redshift and its associated error for each cluster member within R = 1 Mpc for XLSSC 081, XLSSC 085, XLSSC 086, and within R = 500 kpc for XLSSC 082, XLSSC 083, and XLSSC 084.

The redshift of the galaxy identified as the BCG of XLSSC 084 and other additional member candidates (see Table 2) was obtained with NTT+EFOSC2, covering the spectral range from 5000 Å to 9300 Å with an instrumental resolution of 4.1 Å.

The relevant parameters for all the BCGs in the supercluster are shown in Table 3, while the final spectra are shown in Fig. 2.

thumbnail Fig. 3

Final reduced spectra of the BCG for all the clusters discussed here. Spectra have been shifted by arbitrary units to facilitate the viewing.

We calculated the relative velocity of each cluster, assuming as the centre of the structure the BCG of XLSSC 085, the most massive cluster in the structure; we found that the other clusters move with a relative speed between 210 km s-1 and 840 km s-1 with an estimated error of the order of 95 km s-1.

3. Results

From the data sets presented above, we can extract the following results:

  • We are observing a supercluster with a multiplicity of 6 (weobserve six distinct clusters of galaxies, each with its own BCG)with a total extent of 11 × 2.9 Mpc in the sky and 21 Mpc along the line of sight. The total X-ray derived mass is ~ 1.7 × 1015 M⊙, while the gas mass is Mgas = 9.3 × 1013 M⊙. From the total estimated gas mass and total mass of the system, we infer an average gas fraction of ~ 5% in the supercluster; this is typical of what we observe in XXL clusters, see Paper XIII.

  • The optical appearance of four out of the six clusters seems undisturbed and the X-ray emission is centred on the BCG, as can be seen in Fig. 1. On the other hand, XLSSC 082 and, especially, XLSSC 084, show an elongated appearance on the sky, preferentially along the axis XLSSC 082-XLSSC 083-XLSSC 084. The X-ray emission in these three clusters shows a common envelope.

  • Two of the BCGs, XLSSC 082 and XLSSC 084, show a large separation from the X-ray emission centroid at 149 kpc and 202 kpc respectively. An offset between the BCG and the average redshift of the cluster is also evident in the optical data for XLSSC 084 where we measure a velocity difference of 700 km s-1, while this is not observed for XLSSC 082. This likely indicates that XLSSC 084 is in a merging state (see Adami 2000), as also suggested by its disturbed X-ray morphology.

  • An estimate of the crossing time of the supercluster is tc = 2.11 Gyr, while the average escape velocity is of the order of 3.5 × 103 km s-1.

4. Discussion and conclusions

XLSSC-e is currently the most massive and most distant supercluster of galaxies found in XXL. In the literature, starting from the original definition of superclusters (see Bachall 1984) there are several catalogues of superclusters, mostly based on optical data. Only in the last two years (see Chon et al. 2013) has a search for superclusters based purely on X-ray detection been pursued; it reaches out to z ≤ 0.4, and we note that no supercluster with more than three members can be found beyond redshift z ~ 0.2, most likely because of the depth of the RASS survey. XXL is the second survey which has detected several superclusters of galaxies and gone beyond z = 0.4. As already highlighted in Paper II, the selection method used for XXL superclusters has the advantage of relying only on galaxy structures showing clear evidence of a deep potential well and further extend the volume used for such study (z ≥ 0.3). Although a few isolated very high redshift superclusters are known (e.g. Gal et al. 2004), our work is the first attempt to systematically unveil superstructures up to z ≥ 0.5 in a homogeneous X-ray sample.

If we compare our supercluster with the low redshift sample of Chon et al. (2013) we find that its X-ray luminosity (1.7 × 1044 erg s-1 in the 0.1−2.4 keV band relevant for the comparison) is close to the median of that sample; with respect to other supercluster at z ≥ 0.4 (see Verdugo et al. 2012; Geach et al. 2011; Schirmer et al. 2011; Lubin et al. 2009; Kartaltepe et al. 2008; Tanaka et al. 2007) our object has a total mass (M200 obtained using a conversion of r200/r500 = 1.52, Piffaretti et al. 2011) of 2.3 × 1015 M⊙ , again in the middle of the range of the few known objects (see e.g. Table 1 in Schirmer et al. 2011).

Table 3

Properties of the BCGs.

On the other hand, XLSSC-e tends to differ from other known superclusters at those redshifts: instead of having a massive central cluster with infalling filaments and smaller structures, it has almost two equal-sized objects, making it qualitatively different from the network around an already formed massive cluster such as RXJ 1347 (Verdugo et al. 2012). While it is very difficult to infer any dynamical information from such a small number of redshifts, if we put together the relatively small crossing time, the common X-ray emission of three members, and the measured gas fraction and mass, we can speculate that we are observing an un-relaxed structure with an ongoing merging between at least three of the member clusters. If nothing else intervenes to alter the system, and assuming that the estimated crossing time is a good estimate of the merging time, it is likely that the supercluster will have completely merged in ~2.5 Gyr and will resemble a massive cluster of galaxies similar to the most massive known clusters, such as RXJ 1347. The observed gas should be progressively heated up by gravitational collapse and should relax after a few dynamical timescales, thus creating a hot, luminous X-ray halo similar to the ones observed in local massive clusters.

Subsequent extensive spectroscopic follow-up and a kinematic analysis are needed to confirm this hypothesis and to study the galaxy population of this and other large structures discovered by XXL. A study of the surrounding environment of XLSSC-e has been already done by Baran et al. (2016, hereafter Paper IX) using photometric redshifts.

1

XXL-100-GC data are available in computer readable form via the XXL master catalogue browser: http://cosmosdb.iasf-milano.inaf.it/XXL

Acknowledgments

XXL is an international project based around an XMM Very Large Programme surveying two 25 deg2 extragalactic fields at a depth of ~ 5 × 10-15 erg cm-2 s-1 in the [0.5−2] keV band for point-like sources. The XXL website is http://irfu.cea.fr/xxl. Multiband information and spectroscopic follow-up of the X-ray sources are obtained through a number of survey programmes, summarised at http://xxlmultiwave.pbworks.com/. The authors wish to acknowledge the support from the staff at WHT and La Silla; we also thank the French PNCG and the French-Italian PICS for financial support which made this work possible. F.P. acknowledges support from the DLR Verbunforschung grant 50 OR 1117 and from the DFG Transregional Program TR33. N.Ba. and V.Smo. acknowledge the funding by the European Union’s Seventh Frame-workprograms under grant agreements 333654 (CIG, “AGN feedback”) and 337595 (ERC Starting Grant, “CoSMass”). Y.J. acknowledges support by FONDECYT grant No. 3130476.

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All Tables

Table 1

Properties of XLSSC-e supercluster.

In the text
Table 2

Measured redshift and its associated error for each cluster member within R = 1 Mpc for XLSSC 081, XLSSC 085, XLSSC 086, and within R = 500 kpc for XLSSC 082, XLSSC 083, and XLSSC 084.

In the text
Table 3

Properties of the BCGs.

In the text

All Figures

thumbnail Fig. 1

X-ray spectra of the cluster XLSSC 082 taken from a 300 kpc aperture. The best fitting model is also shown. Data from MOS1, MOS2, and pn are plotted in black, red, and green, respectively.
In the text
thumbnail Fig. 2

CFHTLS MegaCam mosaic image in the i band with the X-ray contours superimposed in green; identified X-ray clusters are encircled in magenta circles, which have a radius r = r500, and their XXL IDs indicated. The blue circles show the positions of the BCG in each cluster and the black crosses highlight the point sources excluded from the X-ray analysis. Contour levels are in logarithmic scale and they range from 4.5 counts/s/deg2 to 30 counts/s/deg2. North is up and east is on the left.
In the text
thumbnail Fig. 3

Final reduced spectra of the BCG for all the clusters discussed here. Spectra have been shifted by arbitrary units to facilitate the viewing.
In the text

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