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Planck early results
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Issue
A&A
Volume 536, December 2011
Planck early results
Article Number A26
Number of page(s) 7
Section Cosmology (including clusters of galaxies)
DOI https://doi.org/10.1051/0004-6361/201117430
Published online 01 December 2011

© ESO, 2011

1. Introduction

Very massive clusters above redshift z ~ 1, when the Universe was at half the present age, are predicted to be very rare. They potentially provide a sensitive probe to constrain deviations from the standard ΛCDM paradigm (e.g. Mortonson et al. 2011); e.g., owing to non-Gaussian perturbations, non-standard quintessence models or modified gravity models (see Allen et al. 2011, for a review). They are also ideal targets for studying key aspects of the gravitational physics that drives cluster formation, including measurement of the evolution of the mass concentration. For these reasons, the scientific community has, over the past two decades, put strong effort into the discovery and characterisation of these objects.

thumbnail Fig.1

Left panel: PlanckYSZ map of PLCKG266.6−27.3 obtained with the modified internal linear combination algorithm (MILCA; Hurier et al. 2010) with a spatial resolution of 10′. Middle panel: XMM-Newton exposure-corrected count rate image of the region indicated by the black box in the left panel. It is obtained using data from the EMOS12 and EPN camera in the [0.3−2.0] keV energy band. The contours of the XMM-Newton image after wavelet filtering are overlaid in white. Right panel: corresponding XMM-Newton surface brightness profile. The green line indicates the best-fitting β-model with a cusp (see text); the red line is this model convolved with the point spread function (PSF) of XMM-Newton, and the dashed line is the on-axis PSF of XMM-Newton, normalised to the central intensity. The source is clearly significantly extended.

Until recently it was possible to identify clusters of galaxies only via optical/infrared or X-ray surveys. Indeed, the most distant clusters presently known have all been detected with these techniques, e.g., the IR-selected cluster CLJ1449+0856 at z = 2.07 (Gobat et al. 2011) and the X-ray selected system XMMUJ105324.7+572348 at z = 1.75 (Henry et al. 2010). For both of these objects, extended X-ray emission has been detected with XMM-Newton, confirming their status as fully established galaxy clusters; however, their total masses are more typical of systems in the poor cluster or group regime (≲1014 M⊙). Until recently, the most massive cluster known in the z ≳ 1 universe was XMMUJ2235.3−2557 at z = 1.39, discovered in the XMM-Newton Distant Cluster Project (XDCP) based on serendipitous cluster searches in XMM-Newton observations (Mullis et al. 2005). For this system, Jee et al. (2009) estimate a mass of M200 = (7.3 ± 1.3) × 1014 M⊙ from a weak lensing analysis.

However, clusters are also detectable through the Sunyaev-Zeldovich (SZ) effect (Sunyaev & Zeldovich 1972), the spectral distortion of the cosmic microwave background (CMB) generated via inverse Compton scattering of CMB photons by the hot electrons in the intra-cluster medium. Crucially, the total SZ signal is expected to be closely related to the cluster mass (e.g. da Silva et al. 2004), and its brightness insensitive to redshift dimming. As a result, SZ surveys can potentially provide unbiased cluster samples that are as close as possible to being mass-selected1. They offer an ideal way to identify massive, high-redshift clusters. One recent illustration is the detection of SPT-CLJ2106−5844 at z = 1.13 by the South Pole Telescope (SPT) survey (Foley et al. 2011). With an estimated mass of M200 = (1.27 ± 0.21) × 1015 M⊙, SPT-CLJ2106−5844 is nearly twice as massive as XMMUJ2235.3−2557.

The Planck2 satellite has been surveying the sky in the microwave band since August 2009 (Planck Collaboration 2011a) with a good (band-dependent) spatial resolution of 5 arcmin (Mennella et al. 2011; Planck HFI Core Team 2011a). Compared to other SZ experiments such as ACT (Marriage et al. 2011) or SPT (Carlstrom et al. 2011), Planck brings unique nine-band coverage from 30 to 857 GHz and, most crucially, an exceptionally large survey volume. Planck is the first all-sky survey capable of blindly detecting clusters (i.e., not guided in the search by prior observations), since the ROSAT All-Sky Survey (RASS) in the X-ray domain. This coverage allows detection of the rarest clusters, the most massive objects lying in the exponential tail of the mass function.

Planck Collaboration (2011d) recently published the early SZ (ESZ) sample, the first sample of galaxy clusters detected blindly in the all-sky maps from the first ten months of the Planck survey. The properties of this first sample already show that Planck is detecting previously unknown, massive clusters that do not appear in RASS or in other smaller area SZ surveys (Planck Collaboration 2011e). The ESZ comprises high signal-to-noise ratio (S/N > 6) Planck SZ sources up to z = 0.5. We report here on an SZ source that was blindly identified at S/N ~ 5 in the Planck all-sky survey, and recent XMM-Newton validation observations confirm it is a massive cluster at z ~ 1.

In this paper, we adopt a ΛCDM cosmology with H0 = 70 km s− Mpc, ΩM = 0.3, and ΩΛ = 0.7. The factor is the ratio of the Hubble constant at redshift z to its present-day value. The quantities Mδ and Rδ are the total mass and radius corresponding to a total density contrast δ, as compared to ρc(z), the critical density of the Universe at the cluster redshift; . The SZ flux is characterised by Y500, where  is the spherically integrated Compton parameter within R500 (corresponding to δ = 500), and DA is the angular-diameter distance to the cluster.

2. Planck detection

The blind search for clusters in Planck data relies on a multi-matched filter (MMF) approach (Melin et al. 2006). Candidates then undergo a validation process, including internal quality checks and cross-correlation with ancillary data and catalogues, as described in Planck Collaboration (2011d). This process produces a list of new Planck SZ cluster candidates above a given S/N threshold that require follow-up for confirmation. The XMM-Newton follow-up for validation, undertaken in Director’s Discretionary Time via an agreement between the XMM-Newton and Planck Project Scientists, plays a central role in this confirmation procedure. It consists of snapshot exposures (~10 ks), sufficient for unambiguous discrimination between clusters and false candidates (Planck Collaboration 2011e). The results of the first two runs (completed in September 2010) are reported by Planck Collaboration (2011d,e).

The XMM-Newton validation programme is continuing to explore lower S/N and detection quality criteria. PLCKG266.6−27.3, detected at S/N = 5.03, was observed in the framework of the third run of the XMM-Newton validation programme, for which the analysis is on-going. This run comprises a total of 11 candidates detected at 4.5 < S/N < 5.3 from the same Planck HFI maps used for the construction the ESZ sample. The 11 candidates were sent for scheduling in November 2010 and the observations were performed between 22 December 2010 and 16 May 2011. Interestingly, PLCKG266.6−27.3 has been independently detected in the SPT survey. Its Planck position (, −57°47′29″) is consistent with that of SPT-CLJ0615-5746 (Williamson et al. 2011, published on arXiv.org in January 2011, with a photometric redshift of zphot = 1 ± 0.1).

3. XMM-Newton validation

3.1. Observation and data reduction

PLCKG266.6−27.3 was observed with the XMM-Newton EPIC instrument (Turner et al. 2001; Strüder et al. 2001), using the thin filters and the extended full frame mode for the “pn-CCD” camera. The data analysis and validation procedure is described in Planck Collaboration (2011e). Calibrated event lists were produced with v11.0 of the XMM-Newton Science Analysis System. Data that are affected by periods of high background due to soft proton flares were omitted from the analysis, and the remaining data were pattern-selected and corrected for vignetting, as described in Pratt et al. (2007). Bright point sources were excised from the data. Background treatment is described in Pratt et al. (2010). In the spectroscopic analysis, the cluster component was modelled with an absorbed thermal emission model (mekal) with a hydrogen column density fixed at the 21-cm value of Dickey & Lockman (1990).

The observation, OBSID = 0658200101, was affected by soft proton flares. The net exposure time after flare cleaning is only 2.4 ks for the pn-CCD camera, with a particle background 30% higher than nominal. The MOS camera data are less affected with a clean time of ~12 ks and a background excess about two times lower. We undertook a conservative approach to analysing spectroscopic data, since they are the most sensitive to the background estimate. We first fitted the data from the three cameras simultaneously, then fitted only the MOS cameras. The uncertainties in the physical quantities below reflect the difference in best-fitting values between the two analyses and their errors.

thumbnail Fig.2

XMM-Newton EMOS1 (black) and EMOS2 (red) spectra extracted from a circular region of in radius and centred in the cluster X-ray peak. Only the data points above 2 keV are shown for clarity, but data down to 0.3 keV are used in the spectral fitting. The line is the thermal model for the best-fitting redshift, z = 0.94 ± 0.02. The position of the redshifted Fe K line is marked.

3.2. Confirmation and z estimate

In Fig. 1 we show the vignetting-corrected count rate image of the cluster in the [0.3−2.0] keV band. An extended X-ray source is clearly coincident with PLCKG266.6−27.3. Its total EPIC count rate in the [0.3−2.0] keV band is (0.52 ± 0.02) count/s within 2.3′, the maximum radius of detection. The offset between the X-ray cluster centre, defined as the emission peak at , , and the Planck blind position is , consistent with the position reconstruction uncertainty, driven by the Planck spatial resolution and the source S/N (Planck Collaboration 2011d). The extended nature of the source is confirmed by comparing the surface brightness profile with the XMM-Newton point spread function (PSF) (Fig. 1, right panel). A typical (PSF-convolved) cluster surface brightness model consisting of a β-model with a central cusp (Eq. (2) in Pratt & Arnaud 2002) provides a good fit to the data and further supports the extended nature of the source (Fig. 1).

We extracted a spectrum within a circular region corresponding to the maximum significance of the X-ray detection (). The iron K line complex is clearly detected (Fig. 2). Its significance is 3.6σ, estimated from a fit of the spectrum in the [2−6] keV band with a continuum plus a Gaussian line model. Since the centroid of the line complex depends on the temperature, the redshift is determined from a thermal model fit to the full spectrum, as described in detail in Planck Collaboration (2011e). This yields a precise redshift estimate of z = 0.94 ± 0.02.

4. Physical cluster properties

4.1. An exceptionally luminous and massive cluster

We derived the deprojected, PSF-corrected gas density profile from the surface brightness profile, using the non-parametric method described in Croston et al. (2006). Global cluster parameters were then estimated self-consistently within R500 via iteration about the M500 − YX relation of Arnaud et al. (2010), assuming standard evolution, . The quantity YX, introduced by Kravtsov et al. (2006), is defined as the product of Mg,500, the gas mass within R500, and TX. TX is the spectroscopic temperature measured in the [0.15−0.75] R500 aperture. In addition, L500, the X-ray luminosity inside R500, was calculated as described in Pratt et al. (2009). All resulting X-ray properties, including iron abundance, are summarised in Table 1.

PLCKG266.6−27.3 is an exceptionally luminous system. Its [0.1−2.4] keV band luminosity of (22.7 ± 0.8) ×  1044 erg s-1 is equal to that of the fifth most luminous object in the MCXC compilation of Piffaretti et al. (2011), MACSJ0717.5+3745 at z = 0.55, discovered in the RASS by Edge et al. (2003). Moreover, its [0.5−2.0] keV band luminosity is consistent with that of SPT-CLJ2106−5844, the most luminous cluster known beyond z = 1 (Foley et al. 2011). Collectively, these three clusters are the most luminous systems at z > 0.5. They are only 40% fainter than RXJ1347.5-1145, the most X-ray luminous cluster known in the Universe (Piffaretti et al. 2011).

Table 1

Physical properties of PLCKG266.6−27.3 derived from XMM-Newton data.

Consistent with expectations for high-redshift Planck-detected clusters, we find that this cluster is extremely hot, TX ~ 11 keV, and massive, with a mass of × 1014 M⊙. Our mass estimate is consistent with the less precise value, M500 = 8 ±  2 (statistical) ±  1.9 (systematic) ×  1014 M⊙, which is derived by Williamson et al. (2011) using the relation between SPT S/N and mass. Comparison of the masses of high-redshift systems is not trivial, because the estimation strongly depends on method, e.g. which mass proxy is used and at what reference radius the mass is measured. On the basis of the published mass estimates, PLCKG266.6−27.3 would appear to be the most massive cluster at z ~ 1. Using the same factor to convert M500 to M200 as Foley et al. (2011), we obtain × 1014 M⊙, to be compared to M200 = (12.7 ± 2.1)3 ×  1014 M⊙ for SPT-CLJ2106-5844. However, the last value was derived by combining X-ray and SZ data. A more direct comparison of M500 values estimated from the M500 − YX relation indicates that they are identical within their uncertainties: M500 = (7.8 ± 0.8) × 1014 M⊙ for PLCKG266.6−27.3 and M500 = (9.3 ± 2.0) × 1014 M⊙ for SPT-CLJ2106−5844.

4.2. Y500 Compton parameter versus YX

The MMF blind detection was performed using the universal pressure profile of Arnaud et al. (2010) as a spatial template, leaving the position, characteristic size, θ500, and SZ flux, Y500, as free parameters. The resulting flux, Y500 = (5.6 ± 3.0) ×  10-4 arcmin2, is consistent with the value, ×  10-4 arcmin2, expected from the measured value of YX using the scaling relation derived from the universal pressure profile (Arnaud et al. 2010, Eq. (19)). The cluster size, comparable to Planck’s spatial resolution, is poorly constrained, ± . As discussed in Planck Collaboration (2011d), the uncertainty on the blind Y500 value is then large because of the flux-size degeneracy, where an overestimate of the cluster size induces an overestimate of the SZ signal. The SZ photometry can be improved by using the more precise XMM-Newton position and size in the flux extraction. The Y500 value obtained using these X-ray priors, Y500 = (4.1 ± 0.9) ×  10-4 arcmin2, is lower than the value expected from the X-ray data at the 1.7σ significance level.

Table 2

SZ flux derived from Planck data with the reference value indicated in boldface.

To check the robustness of the Y500 estimate, we compared the MMF value with the one derived from the PowellSnakes (PWS; Carvalho et al. 2009, and in prep.) algorithm and the modified internal linear combination algorithm (MILCA; Hurier et al. 2010). The values are given in Table 2. PWS is a blind detection algorithm that assumes the same profile shape as MMF, but is based on a Bayesian statistical approach, as fully described in Carvalho et al. (2009). MMF and PWS give consistent results, the difference between MMF and PWS Y500 values being about 1.3 times the respective 1σ uncertainties. MILCA is a component separation method that allows reconstruction of the SZ map around the cluster from an optimised linear combination of Planck HFI maps. In contrast to the MMF and PWS methods, the SZ flux derived from MILCA is obtained from aperture photometry, i.e., with no assumptions on SZ profile shape or size. Assuming a typical conversion factor of 2/3 based on the universal profile to convert the total Ytot MILCA measurement to Y500, the MMF and MILCA estimates are in excellent agreement.

Several factors may affect the X-ray and SZ flux measurements and bring them out of accord. We have checked for possible AGN contamination that could lower the Y500 value using the NVSS (at 1.4 GHz, Condon et al. 1998) and SUMSS (at 0.84 GHz, Bock et al. 1999) catalogues, but no bright radio sources are found in the cluster vicinity. The closest radio source with significant flux density is at  away. The source has a 1 GHz flux density of 0.46 Jy. We also find no evidence of radio contamination in the low-frequency Planck bands. On the other hand, the YX measurement may also be increased by AGN contamination, from cluster members or foreground/background galaxies. Point source contamination is difficult to estimate owing to the XMM-Newton PSF. So, we estimate a maximum contribution to the X-ray luminosity from a central active galaxy of ≲ 20%, assuming a point source model normalised to the central value of the X-ray surface brightness. The contribution to the gas mass, hence to YX, would be less, provided that the source is not hard enough to significantly affect TX. Nevertheless, only high-resolution X-ray imaging (e.g., from Chandra) can definitively establish whether X-ray AGNs at the cluster location affect our luminosity or mass measurement. A departure from the universal pressure profile would change the Y500/YX ratio. The density profile shown below does not show any indication of this effect; however, deep spatially resolved XMM-Newton and Chandra spectroscopic observations are needed to derive the radial pressure gradient from the core to R500. A final interesting possibility is that gas clumping could affect the YX measurements. A combination of X-ray and higher resolution SZ observations is required to assess this point.

4.3. Dynamical state and self-similarity of shape up to high z

The available information indicates that PLCKG266.6−27.3 may be particularly dynamically relaxed. The cluster image (Fig. 1, middle panel) does not show any sign of disturbance: the surface brightness is quite regular and quasi-azimuthally symmetric within R500. The offset between the X-ray surface brightness peak and the cluster brightest galaxy (Williamson et al. 2011, Fig. 19) is less than 5″.

To further examine the dynamical state of the cluster, in Fig. 3 we compare its scaled density profile to those of clusters in the local representative X-ray-selected sample REXCESS (Böhringer et al. 2007; Croston et al. 2008). The radii are scaled by R500 and the density by the mean within R500. As extensively discussed by Pratt et al. (2009) and Arnaud et al. (2010), morphologically disturbed (i.e., merging) systems have systematically shallower density profiles than more relaxed cool core objects. This is illustrated in Fig. 3, where we indicate the scaled density profiles of the more relaxed cool core and the dynamically active merging clusters. The scaled density profile of PLCKG266.6−27.3 lies between the two classes, but with an indication of being closer to the relaxed rather than the merging systems. It is thus possible that PLCKG266.6−27.3 is a cool core object at z ~ 1. Such objects are expected to be rare (e.g., Vikhlinin et al. 2007; Santos et al. 2010), and no cluster at this redshift has yet been found to contain a resolved central temperature drop that would confirm the presence of a cool core. A deep exposure at Chandra spatial resolution is needed to check this hypothesis.

It is worth emphasising the similarity beyond the core of the density profile of this cluster with respect to REXCESS systems. This is the first piece of evidence for a similarity of shape up to redshifts as high as z ~ 1.

thumbnail Fig.3

Scaled density profiles of the REXCESS local cluster sample (Böhringer et al. 2007; Croston et al. 2008). The blue lines show the profiles for the cool core systems, and the orange lines the density profiles of the morphologically disturbed systems. The density profile of PLCKG266.6−27.3 is shown with a thick red line.

5. Conclusion

PLCKG266.6−27.3 is the first blindly discovered Planck cluster of galaxies at z ~ 1. It has been confirmed by XMM-Newton in the framework of the on-going validation DDT observations. XMM-Newton data allowed us to measure the redshift with high accuracy (z = 0.94 ± 0.02) and estimate the cluster mass to be M500 = (7.8 ± 0.8) × 1014 M⊙. This XMM-Newton confirmation and redshift estimate is a clear demonstration of the capability of Planck for detecting high-z, high-mass clusters.

PLCKG266.6−27.3 is an exceptional system, both in terms of its luminosity and its estimated mass. Furthermore, unlike other high-redshift clusters, it is likely to be a relaxed system, potentially allowing accurate hydrostatic mass measurements. It is thus a perfect target for deep multi-wavelength follow-up to address such important cosmological issues as the evolution of dark matter profiles, the evolution of the mass-YSZ relation, gas clumping, and the bias between X-ray and lensing mass estimates at such high redshift.

1

In practice, the mass threshold detectable by Planck increases with redshift. The total SZ signal is not resolved by Planck at high z and it decreases with z due to the decreasing angular size of the object.

2

Planck (http://www.esa.int/Planck) is a project of the European Space Agency (ESA) with instruments provided by two scientific consortia funded by ESA member states (in particular the lead countries: France and Italy) with contributions from NASA (USA), and telescope reflectors provided in a collaboration between ESA and a scientific consortium led and funded by Denmark.

3

The error includes an extra ~15% error accounting for uncertainties in the scaling relations.

Acknowledgments

The Planck Collaboration thanks Norbert Schartel for his support of the validation process and for granting discretionary time for the observation of Planck cluster candidates. The present work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). This research has made use of the following databases: SIMBAD, operated at the CDS, Strasbourg, France; the NED database, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration; BAX, which is operated by the Laboratoire d’Astrophysique de Tarbes-Toulouse (LATT), under contract with the Centre National d’Études Spatiales (CNES); and the SZ repository operated by IAS Data and Operation Center (IDOC) under contract with CNES. A description of the Planck Collaboration and a list of its members, indicating which technical or scientific activities they have been involved in, can be found at http://www.rssd.esa.int/Planck_Collaboration. The Planck Collaboration acknowledges the support of: ESA; CNES and CNRS/INSU-IN2P3-INP (France); ASI, CNR, and INAF (Italy); NASA and DoE (USA); STFC and UKSA (UK); CSIC, MICINN and JA (Spain); Tekes, AoF and CSC (Finland); DLR and MPG (Germany); CSA (Canada); DTU Space (Denmark); SER/SSO (Switzerland); RCN (Norway); SFI (Ireland); FCT/MCTES (Portugal); and DEISA (EU).

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All Tables

Table 1

Physical properties of PLCKG266.6−27.3 derived from XMM-Newton data.

In the text
Table 2

SZ flux derived from Planck data with the reference value indicated in boldface.

In the text

All Figures

thumbnail Fig.1

Left panel: PlanckYSZ map of PLCKG266.6−27.3 obtained with the modified internal linear combination algorithm (MILCA; Hurier et al. 2010) with a spatial resolution of 10′. Middle panel: XMM-Newton exposure-corrected count rate image of the region indicated by the black box in the left panel. It is obtained using data from the EMOS12 and EPN camera in the [0.3−2.0] keV energy band. The contours of the XMM-Newton image after wavelet filtering are overlaid in white. Right panel: corresponding XMM-Newton surface brightness profile. The green line indicates the best-fitting β-model with a cusp (see text); the red line is this model convolved with the point spread function (PSF) of XMM-Newton, and the dashed line is the on-axis PSF of XMM-Newton, normalised to the central intensity. The source is clearly significantly extended.
In the text
thumbnail Fig.2

XMM-Newton EMOS1 (black) and EMOS2 (red) spectra extracted from a circular region of in radius and centred in the cluster X-ray peak. Only the data points above 2 keV are shown for clarity, but data down to 0.3 keV are used in the spectral fitting. The line is the thermal model for the best-fitting redshift, z = 0.94 ± 0.02. The position of the redshifted Fe K line is marked.
In the text
thumbnail Fig.3

Scaled density profiles of the REXCESS local cluster sample (Böhringer et al. 2007; Croston et al. 2008). The blue lines show the profiles for the cool core systems, and the orange lines the density profiles of the morphologically disturbed systems. The density profile of PLCKG266.6−27.3 is shown with a thick red line.
In the text

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