LETTER TO THE EDITOR

XMMU J100750.5+125818: a strong lensing cluster at z = 1.082^,,

A. D. Schwope¹ - G. Lamer¹ - A. de Hoon¹ - J. Kohnert¹ - H. Böhringer² - J. P. Dietrich³ - R. Fassbender² - J. Mohr^4,2,5 - M. Mühlegger² - D. Pierini² - G. W. Pratt^6,2 - H. Quintana⁷ - P. Rosati⁸ - J. Santos⁹ - R. Suhada²

1 - Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2 - Max-Planck-Institut für Extraterrestrische Physik, 85748 Garching, Germany
3 - Physics Department and Michigan Center for Theoretical Physics, University of Michigan, 450 Church St, Ann Arbor, MI 48109-1040, USA
4 - Department of Physics, Ludwig-Maximilians-Universität, Scheinerstraße 1, 81679 Munich, Germany
5 - Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
6 - Laboratoire AIM, IRFU/Service d'Astrophysique - CEA/DSM - CNRS - Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
7 - Departamento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago, 22, Chile
8 - European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
9 - INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy

Received 26 February 2010 / Accepted 1 April 2010

Abstract
We report on the discovery of the X-ray luminous cluster XMMU J100750.5+125818 at redshift 1.082 based on 19 spectroscopic members. The cluster displays several strong lensing features. SED modeling of the lensed arc features from multicolor imaging with the VLT and the LBT reveals likely redshifts 2.7 for the most prominent of the lensed background galaxies. Mass estimates are derived for different radii from the velocity dispersion of the cluster members,   10¹⁴ , from the X-ray spectral parameters,   10¹⁴ , and the largest lensing arc,   10¹³ . The projected spatial distribution of cluster galaxies appears to be elongated, and the brightest galaxy lies off center with respect to the X-ray emission, indicating a not yet relaxed structure. XMMU J100750.5+125818 offers excellent diagnostics of the inner mass distribution of a distant cluster with strong and weak lensing, in combination with optical and X-ray spectroscopy.

Key words: galaxies: clusters: individual: XMMU J100750.5+125818 - X-rays: galaxies: clusters

1 Introduction

A crucial prerequisite using clusters of galaxies for cosmological studies is knowing the scaling relations between cluster mass and an observable proxy, such as X-ray luminosity or X-ray gas temperature. These relations must be calibrated at varying redshifts, using theoretical modeling and hydrosimulations or simply empirical relations. Clusters at low and intermediate redshifts are bright enough in X-rays to measure the total gravitating mass by deprojecting their temperature and density profiles. This is currently difficult for clusters beyond z  1 and subject to large statistical errors (e.g. Rosati et al. 2004). In this high-redshift regime, independent mass estimates based on gravitational lensing (both in the weak and the strong regimes) have long been invoked as the most effective method for calibrating cluster masses. In particular, mass measurements based on multiple strong-lensing features are highly robust. Although such measurements are only possible in the central region of a cluster, they are independent of model asssumptions and can help to break the mass-sheet degeneracy of parameter-free weak-lensing mass-reconstructions.

To obtain a well-defined sample of clusters at high redshift, z > 0.8, we started the XDCP (XMM-Newton Distant Cluster Project, Mullis et al. 2005; Böhringer et al. 2005; Fassbender et al. 2008) based on archival XMM-Newton X-ray data. The XDCP has been very successful, so far providing 18 clusters at z>0.8 and 10 clusters with z>1 including five redshift confirmations from other projects (for selected results see e.g. Rosati et al. 2009; Santos et al. 2009; Fassbender et al. 2008).

Here we report the discovery of a distant strong-lensing cluster of galaxies at redshift z=1.082 that shows a giant arc and other arc-like features interpreted as the gravitationally lensed images of more distant, background objects. We study the properties of the lensing cluster and the lensed galaxies on the basis of available optical and X-ray imaging and spectroscopy.

Throughout this paper we use a standard CDM cosmology with parameters H₀=71 km s^-1 Mpc^-1, , and , which gives a scale of 8.188 kpc arcsec^-1 at redshift 1.082. All magnitudes in this paper are given in the AB system.

	Figure 1: ( Left) UBVR image of the field around 1007 indicating X-ray contours and tentatively identified lensing features. The cross indicates the X-ray position and the square the size of the color image shown to the right. (Right) Color composite of the central region of 1007 with mean (Rz)V(UB) images in the RGB channels. North is at the top and east to the left in both frames.
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2 Observations, analysis, and results

2.1 XMM-Newton X-ray observations

XMMU J100750.5+125818 (hereafter 1007) was found serendipitously as an extended X-ray source in our systematic search for distant clusters of galaxies. The cluster candidate was detected in a field with a nominal exposure time of 22.2 ks at an off-axis angle of 11.7' (OBSID: 0140550601). Following Pratt & Arnaud (2003), time intervals with high background were excluded by applying a two-step flare-cleaning procedure, first in a hard energy band (10-14 keV) and subsequently in the 0.3-10 keV band, after which 21.4 ks of clean exposure time remained for the two MOS cameras and 18.0 ks for the PN instrument.

Images and exposure maps were created in the 0.35-2.4 keV detection band, which was chosen to maximize the signal-to-noise ratio of the X-ray emission of massive galaxy clusters at z>0.8 compared to the background components (Scharf 2002). Source detection was performed with the SAS task eboxdetect followed by the maximum likelihood fitting task emldetect for determining source parameters. 1007 was found as an extended X-ray source with low surface brightness at position , with a total of about 200 source photons and a core radius of 24'' at a significance level of DET and an extent likelihood of EXT . 1007 is not listed in the 2XMM catalog (Watson et al. 2009), since its detection likelihood after the first step of the detection chain (eboxdetect) remained below the threshold for performing the second step (emldetect). It was revealed as a cluster candidate in this work, however, owing to the use of a different detection band and by lowering the detection threshold to perform emldetect.

2.2 Optical imaging with VLT/FORS2 and LBT/LBC

Neither DSS nor the SDSS revealed a convincing counterpart to the unique X-ray source 1007. The field containing 1007 was thus observed with FORS2 at the VLT in January/February 2007 through ESO filters R_SPECIAL+76 and z_GUNN+78 with total integration times of 1920 s and 960 s, respectively. The combination of filters was chosen to allow an unambiguous redshift determination up to through the identification of a cluster red sequence (CRS).

All the imaging data of this paper were reduced with an AIP-adaptation of the GaBoDS-pipeline described in Erben et al. (2005). It comprises all the pre-processing steps (bias- and flatfield-, and fringe-correction), super-flat correction, background subtraction, and creation of a final mosaic image using SWarp and ScAMP (Bertin 2006).

The photometric calibration of the R-band image was achieved through observations of Stetson (2000) standard fields, the photometry of the z-band image was tied to the SDSS. The nonstandard z-band cut-on filter in use at the VLT leads to an estimated systematic calibration uncertainty of 0.05 mag.

The measured image seeing of the VLT R- and z-band images is  and  , respectively. Object catalogs were generated with SExtractor (Bertin & Arnouts 1996) in double image mode. The magnitudes of catalogued objects were corrected for Galactic foreground extinction. From the observed drop in the number-flux relations with respect to a power-law expectation we estimate a 50% completeness of the catalogs of and . Following Mullis et al. (2005, and its erratum), we used fixed apertures with diameter 3.5   to determine object colors.

A mere two-band, false-color composite of the VLT imaging data proved enough to locate an overdensity of red, early type galaxies at the position of the extended X-ray emission. Color- and location-selected possible cluster members form a red sequence in the color-magnitude diagram of Fig. 2. The red sequence color was estimated from the average color of all galaxies within 30 of the X-ray position (see box in Fig. 2), R - z = 1.91, which hints at a cluster redshift beyond 0.9.

Further imaging observations were performed with the Large Binocular Telescope (LBT) equipped with the Large Binocular Camera (LBC) through Bessel B,V filters, and the  filter. Table A.1 lists the details of the LBT imaging observations. The photometric zeropoints for the LBC images were derived by using the SDSS photometry of stellar objects in the field.

A multi-color composite of the center of the field including VLT/FORS2- and LBT/LBC-data is shown in Fig. 1 with X-ray contours overlaid. Residual astrometric uncertainties in the XMM-Newton X-ray image were removed on the basis of identified X-ray point sources in the FORS- and LBT-images, respectively.

The cluster has its brightest galaxy (BCG) at position and between structures B1 and B2 (see Fig. 1), i.e., away from the apparent center of the galaxy distribution by about 20'' and away from the X-ray position. The BCG has an apparent magnitude   0.06, an observed color   0.08, and an absolute magnitude .

	Figure 2: Color-magnitude diagram of 1007. The size of the symbols encodes vicinity to the X-ray center of the cluster (within 30 and 60 and beyond). Objects within the dotted box were used to estimate the color of the red sequence (solid horizontal line). Objects framed with squares are spectroscopically confirmed cluster members. The shaded region top right indicates >50% incompleteness.
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The spatial distribution of the galaxies that form the red sequence in Fig. 2 appears stretched in a band from SSE to NNW with a centroid about 10'' away from the centroid of the cluster X-ray emission. However, the best-fit X-ray position (maximum likelihood fit of an assumed -profile folded with the PSF of the X-ray telescope) lies well within the galaxy assembly (see Fig. 1).

2.3 Optical spectroscopy

Multi-object spectroscopy of 1007 was performed with VLT/FORS2 using the MXU option in December 2007 and January 2008. One mask was prepared to target 37 candidate objects selected as possible cluster members based on color and stellarity index. Individual exposures of 1308 s were combined to yield a total integration time per object of 2.9 h. Spectra were obtained with grism 300I. They cover the wavelength range 6000-11 000 Å with a scale of 3.2 Å/pixel at the chosen 2   2 binning.

We obtained 32 classifiable spectra (among them two late-type stars) and measured the redshifts of galaxies by fitting double Gaussians to the Ca H&K lines. A fixed wavelength ratio and the same width was used for the two Gaussians. The measured redshifts, together with coordinates, brightness, and color of the 32 objects, are listed in Table A.2 and marked in Fig. A.3 (Appendix).

We find 19 concordant redshifts between 1.075 and 1.088 with a weighted mean of 1.08103 and a median of 1.08207. We assume a cluster redshift of 1.082 in the following. The redshifts in this interval are Gaussian distributed, a one-sample KS-test reveals a probability of rejecting the null hypothesis of 1.3%. The distribution of all redshifts is shown in Fig. A.1. Following Danese et al. (1980), we calculated a line-of-sight velocity dispersion of  km s^-1 with a 90% confidence range between 437 km s^-1 and 780 km s^-1.

Although the cluster does not appear to be relaxed, we may estimate a mass on the virial assumption. Using   (Carlberg et al. 1997), we obtain  kpc, and   where the given errors correspond to the 90% confidence interval of the velocity dispersion. Using the dark-matter halo, virial scaling relation by Evrard et al. (2008) instead, one obtains  .

2.4 X-ray spectroscopy

X-ray spectra were extracted from the calibrated photon event lists of all three EPIC cameras onboard XMM-Newton. An aperture with 60  radius (500 kpc at the cluster distance) was used to extract source and background photons. The X-ray spectrum of 1007 contains 200 net photons in the full XMM-band after background subtraction.

An attempt was made to constrain the plasma temperature with a thin thermal plasma model (a MEKAL model in the X-ray spectral fitting package XSPEC, Mewe et al. 1985). Fit optimization was achieved by using the Cash-statistic with one count per bin (Krumpe et al. 2008). It does not, however, provide a goodness-of-fit statistic. The column density of absorbing matter was fixed to its galactic value,   10²⁰ cm^-2, the metal abundance to , and the redshift to z=1.082.

Despite fixing these parameters, the X-ray temperature could only be roughly constrained. The fit shown in Fig. A.2 converges at a temperature of T = 5.7 keV (kT > 2.05 keV with 90% confidence, the temperature is not constrained towards high values) implying a bolometric X-ray luminosity of   10⁴⁴ erg s^-1 (   10⁴⁴ erg s^-1 at 90% confidence,   10⁴³ erg s^-1).

Following Pratt et al. (2009), we estimated the cluster mass using the luminosity-mass scaling relation. We used their BCES orthogonal fit to the Malmquist-bias corrected L-M relation, assumed self-similar evolution,  h(z)^-7/3, and obtained M₅₀₀ = 1.0  10¹⁴  (assuming our aperture covers R₅₀₀). If we instead use the temperature-mass relation of Vikhlinin et al. (2009), we find M₅₀₀=2.1  10¹⁴  with a lower limit of  M₅₀₀=4.3  10¹³ .

2.5 Lensing properties

Our follow-up imaging observations confirms the existence of lensing arcs and further lensing features in the image. We used the imaging data in the five passbands (UBVRz) from the LBT and the VLT to calculate photometric redshifts of the lensed background objects. Owing to the distorted morphology of the lensing arcs we manually defined apertures matching the shape of each feature. The fluxes of the objects were then extracted using the same aperture in each image and converted to AB magnitudes.

Table 1:   Photometric redshifts of lensing features, with all redshifts forced to be larger than the cluster redshift.

We used the publicly available hyperz code (Bolzonella et al. 2000) to compare the spectral energy distributions of the lensing features with the synthetic galaxy SEDs from Bruzual & Charlot (1993). The parameters for the construction of the SED data cube were galaxy class, star-forming age, internal reddening, and redshift. For the lens features we only allowed redshifts beyond the redshift of the cluster (z=1.082) and combinations of age and redshift consistent with our adopted cosmological parameters.

Table 1 shows the results of the SED fitting for the image components as indicated in Fig. 1. Column (2) gives the best-fitting photometric redshift and Col. (3) the 90% confidence limits of the redshift within the best fitting SED template.

The resulting photometric redshifts confirm our tentative identification of multiple-lensed components. Components A1/ A2 and C1/C2 are likely to be images of the same background objects at and , respectively. The situation is less clear for components B1-B3, where the uncertainties in the photometric redshifts are large.

We estimate a lensing mass assuming a circularly symmetric lens (a special case is the singular isothermal sphere - SIS). The projected mass inside a tangential arc then becomes    . The effective distance D becomes  Mpc for and .

Difficulties arise from the faintness of the lensing features and from the badly determined cluster center. A trace of the feature A1 implies an Einstein radius of and a corresponding mass of   10¹³  (a 15% uncertainty in radius is assumed). If one uses the distance between A1 and the X-ray center ( ) instead, the mass inside this ring becomes 5.7  10¹³ .

3 Discussion and conclusions

We have presented results of an initial study of the X-ray selected cluster 1007. We determine a redshift of z=1.082 based on 19 spectroscopic confirmed members. As the most prominent property, the cluster shows several strong lensing features. It is an optically rich cluster; and the absence of a dominant BCG and the elongated distribution of member galaxies suggest that it is a not yet relaxed structure. The bolometric X-ray luminosity is    .

Estimates of the cluster mass were obtained via strong lensing, X-ray spectroscopy, and the velocity dispersion of member galaxies. The values obtained at different radii are summarized in Fig. 3. One obtains overlapping error bars for the mass at R₂₀₀ (virial radius) and at R₅₀₀ (X-ray aperture) because of insufficient data and uncertainties in the scaling relations. If one takes   10¹⁴  at face value, our strong-lensing result seems to be discrepant with an NFW-profile. All data can be made compliant at a mass of about 4  10¹⁴ , higher than - but not excluded by - the velocity dispersion and the X-ray temperature, whereas a weak-lensing mass is key to fixing the halo profile at large radii, and a deep X-ray observation is necessary to determine the X-ray morphology and the gas temperature. The modeling of the strong-lensing system is important for probing the inner density profile.

Figure 3: Comparison of different mass estimates for 1007 in units of 1014 . Labels indicate results based an scaling relations by Evrard et al. (2008); Vikhlinin et al. (2009); Pratt et al. (2009); Carlberg et al. (1997). The dashed and solid lines indicate universal density profiles (Navarro et al. 1997) with concentration parameters of 3.86 and 3.45 for virial masses 1.8 and 4.6  1014  at an assumed  kpc, respectively (Bullock et al. 2001).

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Interestingly, in a study of a complete sample of 12 MACS clusters, Zitrin et al. (2010) find the observed Einstein radii to be larger, hence the central density of the clusters higher than predicted by simulations, and interpret their finding as a challenge to cluster formation in a CDM model. They invoke the possibility that the formation of clusters started at earlier epochs than currently assumed, leading to higher central dark matter concentrations. Also, Jee et al. (2009) suggest this scenario as a possible explanation for the discovery of unexpectedly massive clusters at 1 in the moderate survey volume probed by the XMM-Newton serendipitous surveys (such as XMMU J2235-2557 and 2XMM J083026+524133, Rosati et al. 2009; Mullis et al. 2005; Lamer et al. 2008). However, in a recent study of the strong lensing clusters in the M ARE N OSTRUM U NIVERSE, Meneghetti et al. (2010) find the concentration and the X-ray luminosity to be biased high and find an excess of kinetic energy within the virial radius among the strong lensing clusters. 1007 is a target highly suited to comparing theory and observation.

The elongated distribution of the cluster member galaxies may point to the principal direction of merging or accretion of 1007 (Dubinski 1998). The merging hypothesis is consistent with the system being a strong lens (see e.g. the chain distribution of the brightest member galaxies in the strong-lensing main component of the Bullet Cluster, Bradac et al. 2006). On the other hand, the chain distribution of the bright member galaxies of 1007 may suggest that the infall of these galaxies happens along filaments with small impact parameters, so that dynamical friction is particularly efficient in dragging them to the cluster center and originating a dominant BCG within a short time (D'Onghia et al. 2005). In analogy with the formation of fossil galaxy groups studied by these authors, the small impact parameters of the filaments may lead to an early assembly of the gas in the center, thus to an early start of its cooling. The resulting system may exhibit an enhanced X-ray luminosity with respect to the optical one. This efficient, early formation may offer an alternative explanation to merging as the reason X-ray luminous clusters with chain distribution of their bright members in particular are detected at high redshifts.

We thank J. Wambsganss for providing software code describing the lensing geometry. We thank S. Schindler and W. Kausch for an early analysis of the lensing features. H.Q. thanks the FONDAP Centro de Astrofisica for partial support. This work was supported by the DFG under grants Schw 536/24-2 and BO 702/16-3, and the German DLR under grant 50 QR 0802. We thank our referee, Florence Durret, for constructive criticism. The work was based on data acquired using the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are: the University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and the Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and the University of Virginia.

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Online Material

Appendix A: Supplementary electronic material

This Appendix gives more detailed information about optical observations, the X-ray spectrum, the redshift distribution, and the brightness of spectroscopically classified objects in the field of 1007.

Table A.1:   LBT image parameters.

	Figure A.1: Redshift distribution of galaxies in the field of 1007 observed with VLT-FORS2-MXU (see Table A.2). The inset is a blow-up comprising the redshift range covered by likely cluster members.
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	Figure A.2: XMM-Newton X-ray spectrum of 1007 (EPIC-pn in black, EPIC-MOS in red and green).
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Table A.2:   Spectroscopically confirmed cluster member galaxies (upper part) and nonmember galaxies targeted by VLT-MOS spectroscopy sorted by increasing z-band magnitude.

	Figure A.3: VLT/FORS2 R-band image with slit positions identified (north is top, east to the left). Labels are the same as in the first column of Table A.2. Likely cluster members are encircled, unrelated classified objects (lower section of Table A.2) are framed with squares, objects that were positioned on slits but could not yet be classified are framed with rhombs.
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Footnotes

... 1.082

Based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under programs 78.A-0265 and 80.A-0659.

...

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

...

Appendix is only available in electronic form at http://www.aanda.org

...

DET, EXT: detection likelihood and extent likelihood of the source, , where P is the probability that the detection (the extent) is spurious due to a Poissonian fluctuation.

...1007

We regard the NVSS radio source NVSS J100751+125901 at  , (J2000.0), marked in Fig. 1 and confirmed to be point-like by FIRST ( , ) with an integrated flux of 5.59  0.19 mJy (positional offset of  ) to be unrelated to the X-ray source.

All Tables

Table 1:   Photometric redshifts of lensing features, with all redshifts forced to be larger than the cluster redshift.

Table A.1:   LBT image parameters.

Table A.2:   Spectroscopically confirmed cluster member galaxies (upper part) and nonmember galaxies targeted by VLT-MOS spectroscopy sorted by increasing z-band magnitude.

All Figures

	Figure 1: ( Left) UBVR image of the field around 1007 indicating X-ray contours and tentatively identified lensing features. The cross indicates the X-ray position and the square the size of the color image shown to the right. (Right) Color composite of the central region of 1007 with mean (Rz)V(UB) images in the RGB channels. North is at the top and east to the left in both frames.
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In the text

	Figure 2: Color-magnitude diagram of 1007. The size of the symbols encodes vicinity to the X-ray center of the cluster (within 30 and 60 and beyond). Objects within the dotted box were used to estimate the color of the red sequence (solid horizontal line). Objects framed with squares are spectroscopically confirmed cluster members. The shaded region top right indicates >50% incompleteness.
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In the text

	Figure 3: Comparison of different mass estimates for 1007 in units of 1014 . Labels indicate results based an scaling relations by Evrard et al. (2008); Vikhlinin et al. (2009); Pratt et al. (2009); Carlberg et al. (1997). The dashed and solid lines indicate universal density profiles (Navarro et al. 1997) with concentration parameters of 3.86 and 3.45 for virial masses 1.8 and 4.6  1014  at an assumed  kpc, respectively (Bullock et al. 2001).
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In the text

	Figure A.1: Redshift distribution of galaxies in the field of 1007 observed with VLT-FORS2-MXU (see Table A.2). The inset is a blow-up comprising the redshift range covered by likely cluster members.
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In the text

	Figure A.2: XMM-Newton X-ray spectrum of 1007 (EPIC-pn in black, EPIC-MOS in red and green).
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In the text

	Figure A.3: VLT/FORS2 R-band image with slit positions identified (north is top, east to the left). Labels are the same as in the first column of Table A.2. Likely cluster members are encircled, unrelated classified objects (lower section of Table A.2) are framed with squares, objects that were positioned on slits but could not yet be classified are framed with rhombs.
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In the text