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A&A
Volume 527, March 2011
Article Number L10
Number of page(s) 6
Section Letters
DOI https://doi.org/10.1051/0004-6361/201016169
Published online 08 February 2011

© ESO, 2011

1. Introduction

The “redshift desert” at z > 1.5 has until recently also been known as a “galaxy cluster desert”, owing to the difficulty in detecting these systems at these redshifts and, in particular, the challenging spectroscopic cluster confirmation once the 4000 Å-break is redshifted to beyond 10 000 Å. Kurk et al. (2009) reported on a proto-cluster-like structure at z = 1.6 in the deep GMASS spectroscopic survey without detectable extended X-ray emission in the Mega-second exposure of the Chandra Deep Field South. Papovich et al. (2010) and Tanaka et al. (2010) independently confirmed an infrared (IR) selected system at z = 1.62 in a deep survey field, and a subsequent detection of its weak X-ray emission consistent with a total mass estimate in the group regime (M200 ≃ 6 × 1013   M⊙). Similar approximate system masses were reported for the first X-ray selected cluster/group at z = 1.75 in the XMM-Newton Lockman Hole pencil-beam survey by Henry et al. (2010) and the IR-selected system CL J1449+0856 at z = 2.07 in the “Daddi Field” by Gobat et al. (2010), which subsequently showed indications of very weak extended X-ray emission.

thumbnail Fig. 1

Optical/NIR appearance of the galaxy cluster XMMU J1007.4+1237 at z = 1.555 with XMM-Newton X-ray contours overlaid in cyan in the logarithmically spaced range [0.014, 0.074] counts arcsec-2 (0.35–2.4 keV). Left: z + H-band color composite (2.3′ ×  2.3′) centered on the cluster. Spectroscopic cluster members are marked by white circles, 30″ (dashed) and 60″ (solid) radii around the X-ray center are indicated in yellow. Right: H-band zoom on the cluster core region. Red (IDs 1 and 2) and blue starburst (IDs 3 and 4) cluster members are marked by small circles, spectroscopically confirmed foreground galaxies are indicated by green crosses, and the outer dashed circle represents the 30″ radius.

Observational studies of galaxy clusters at the highest accessible redshifts provide a direct view of the early assembly phase of the most massive dark matter (DM) halos, the X-ray emitting hot intracluster medium (ICM), and their galaxy populations. In contrast to the dominating “red and dead” galaxies in local clusters with a well-defined red-sequence of quenched, passively evolving early-type galaxies (ETGs), dramatic changes are expected as the main star-formation epoch of the bulk of the cluster galaxies is approached. At z > 1.5, corresponding to lookback times of  ≳ 9.3 Gyr, we are close to the global cosmic star formation (SF) density peak (Hopkins & Beacom 2006). At this epoch, hydrodynamical simulations suggest that the SF quenching effects in dense environments should vanish as the specific star formation densities in clusters approach the peak values in the field (e.g. Romeo et al. 2005). Recent studies of XMMXCS J2215.9-1738 at z = 1.46 indeed found strong ongoing SF activity all the way to the central cluster core region (Hayashi et al. 2010; Hilton et al. 2010) and for CL J1449+0856 at z = 2.07 a tight cluster red-sequence cannot be discerned (Gobat et al. 2010). However, the almost completely quenched star formation at the core of the very massive cluster XMMU J2235.3-2557 at z = 1.39 (Strazzullo et al. 2010; Rosati et al. 2009) supports the idea that there was accelerated galaxy evolution and early SF suppression in the more massive systems.

Here we report on the discovery of a new bona fide galaxy cluster in the “redshift desert”. The system XMMU J1007.4+1237 at z = 1.555 was blindly (i.e. without optical/IR data) detected as a serendipitous extended X-ray source within the XMM-Newton Distant Cluster Project (XDCP, Böhringer et al. 2005; Fassbender 2007), a systematic 80 deg2 archival search for X-ray luminous systems at z > 0.8. Section 2 of this Letter presents the X-ray data, near-infrared (NIR) imaging, and spectroscopic observations, followed by a discussion in Sect. 3 and conclusions in Sect. 4. We assume a ΛCDM concordance cosmology with (H0, Ωm, ΩDE, w) = (70 km s-1Mpc-1, 0.3, 0.7,  − 1). A redshift of z = 1.555 corresponds to a lookback time of 9.40 Gyr and a projected physical scale of 8.47 kpc/″.

2. Observations, data analysis, and results

2.1. XMM-Newton X-ray selection and Chandra cross-check

The original XDCP source detection run was applied to a total of 470 XMM-Newton archival fields and performed with SAS v6.5 using the tasks eboxdetect and emldetect for a sliding box detection with a subsequent maximum likelihood analysis. The X-ray source associated with XMMU J1007.4+1237 was detected at an off-axis angle of 10.7′ in a medium deep observation of 22.2 ks (OBSID: 0140550601) targeting the AGN PG 1004+130. After applying a strict two-level cleaning process to remove periods of enhanced solar flare activity, 18.0 ks (21.5 ks) of data remained for the PN (MOS) cameras. On the basis of these data, the cluster was characterized as a compact extended source (rc ≃ 7″) with a 4σ extent significance1 based on about 130 source counts (0.3–4.5 keV).

To achieve a more accurate determination of source parameters, we re-reduced the XMM-Newton data with SAS v10.0.0 and applied the growth curve analysis (GCA) method of Böhringer et al. (2000) in the soft 0.5–2 keV band. We measure an unabsorbed 0.5–2 keV band flux in the estimated R500 aperture ( ≃ 42″) of fX,   500 ≃ (5.6 ± 1.1) × 10-15 erg s-1 cm-2. Using the redshift information of Sect. 2.3, we find a luminosity of LX,   500 ≃ (8.6 ± 1.7) × 1043 erg/s in the soft 0.5–2 keV band, corresponding to a bolometric ICM energy output of LX, 500bol≃(2.1±0.4)×1044\hbox{$L^{\mathrm{bol}}_{\rm X,~500}\simeq(2.1 \pm 0.4)\times 10^{44}$} erg/s for the estimated TX quoted below.

Figure 1 shows the logarithmically spaced X-ray contours based on the adaptively smoothed X-ray surface brightness (SB) distribution of the cluster with significance levels spanning 2–12 σ above the mean background, corresponding to 0.014–0.074 net source counts arcsec-2 in the 0.35–2.4 keV band. Owing to the intrinsically compact nature and the faintness of the source, we cannot rule out residual point-source contributions to the measured luminosity at this point. Under the assumption that the thermal ICM emission dominates, we estimate an X-ray luminosity-based total cluster mass of M200est≃2×1014 M⊙\hbox{$M^{\mathrm{est}}_{200}\simeq2\times10^{14}\,M_{\sun}$} following the approach of Fassbender et al. (2010) and a corresponding ICM temperature of TXest≃4.3\hbox{$T^{\mathrm{est}}_{\rm X}\simeq4.3$} keV using the M − T relation of Arnaud et al. (2005).

The target PG 1004+130 was also observed with Chandra for 41.6 ks, placing XMMU J1007.4+1237 at an off-axis angle of 11′. After a standard reduction, we could find the cluster source with a total of about 80 counts within R500 with both celldetect (2.8σ) and wavdetect (5.6σ). However, at this off-axis angle the Chandra PSF ( ≃ 6″) is only marginally better than the XMM-Newton resolution, thus allowing only a consistency check at this point. Figure 2 shows the log-spaced Chandra SB contours of the adaptively smoothed 0.7–2 keV band image in blue covering a dynamic range of 0.014–0.082 source counts arcsec-2. We can confirm the principal cluster extent direction along the east-west axis and find a more pronounced main flux peak in close proximity to the brightest cluster galaxy (BCG).

2.2. Near-infrared follow-up imaging

For the optical counterpart identification we obtained medium deep H and z-band imaging data with the prime-focus wide-field (15.4′ × 15.4′) near-infrared (NIR) camera OMEGA2000 at the Calar Alto 3.5m telescope. First observations were performed on 4 January 2006 in the H-band (50 min) in moderate to poor conditions with 1.1–1.6″ seeing, complemented by additional data in photometric conditions on 7 January 2007 in H (30 min) and z-band (50 min) with 0.9–1.3″ seeing. The data were reduced with a designated OMEGA2000 NIR pipeline (Fassbender 2007) and co-added to deep image stacks after inspection of all individual frames. The final stacks comprise 63 min of clean data in H (1.39″ FWHM) and 28 min in z (1.17″ FWHM), after rejection of frames with low atmospheric transparency in H (12%) and corrupted frames in z (44%). This results in 50%-completeness limits (Vega) of Hlim ~ 20.7 and zlim ~ 22.5. Dual-image band photometry with the H stack as a detection frame was performed with Sextractor (Bertin & Arnouts 1996). The photometric calibration was tied to 2MASS point sources (Cutri et al. 2003) in H and designated standard star observations in z (Smith et al. 2002), which was cross-checked with SDSS photometry in the science field.

Figure 1 shows the z + H color composite image2 of the cluster (left) and a H-band zoom on the core region (right) with the XMM-Newton X-ray contours overlaid (Sect. 2.1). The larger-scale cluster environment with surface density contours of very red galaxies in the range 2–16 arcmin-2 is displayed in Fig. 2 (red). The z − H versus H color magnitude diagram (CMD) of the field is presented in the top panel of Fig. 3. Although the current photometric depth is limited, reaching m ∗  +0.7 in the detection band, a fair initial redshift estimate for the cluster of z ≃ 1.7 ± 0.2 (Fassbender 2007) could still be obtained prior to spectroscopy based on the comparison of the observed color of the reddest central objects with simple stellar population (SSP) models (Fig. 3, horizontal lines).

thumbnail Fig. 3

Top: z − H vs. H CMD of the cluster field. Galaxies with projected cluster-centric distances of  < 30″ (30–60″) are displayed in red (green). Secure spectroscopic cluster members are marked by open squares, foreground objects are crossed-out. The vertical blue line indicates the apparent H magnitude of an L ∗  galaxy at z = 1.56, the black line shows the H-band magnitude limit. Two solar metallicity SSP models for formation redshifts of zf = 5 (red dashed line) and zf = 3 (blue dashed) are overplotted. Bottom: VLT/FORS 2 spectra of three secure cluster members smoothed with a 7 pixel boxcar filter, associated IDs (1, 3, 4) refer to Table 1. Redshifted spectral features are indicated together with an overlaid LRG template spectrum in blue (ID 1), sky emission lines (bottom) and telluric absorption (top) are shown in red.

2.3. Spectroscopic confirmation and radio properties

For the final spectroscopic confirmation of the cluster, we obtained deep observations with VLT/FORS 2 (Program ID: 081.A-0312) using a single MXU-mode (Mask eXchange Unit) slit-mask (field-of-view 6.8′ × 6.8′). A total of five 1 h observing blocks were executed targeting XMMU J1007.4+1237 in good seeing conditions of 0.8″ on 25 April 2008 and 5 May 2008. The chosen instrument setup with the 300 I grism without blocking filter and a slit width of 1″  provides a wavelength coverage of 5800–10 500 Å with a resolution of R = 660. The resulting ten individual frames with 21.8 min net exposure times each (3.6 h in total) were reduced with a new FORS 2 adaptation of the VIMOS Interactive Pipeline and Graphical Interface (VIPGI, Scodeggio et al. 2005), which includes bias subtraction, flat field corrections, image stacking, extraction of background-subtracted 1D spectra, and wavelength calibration by means of a helium-argon lamp reference line spectrum (Nastasi et al., in prep.). The final stacked spectra were corrected for the sensitivity function of the FORS 2 instrument and then cross-correlated with a galaxy template library for a semi-automated redshift determination using the IRAF package RVSAO (Kurtz & Mink 1998) and EZ (Garilli et al. 2010).

Owing to the faintness of the sources and the limited photometric information, the primary slit targets were red galaxies (z − H > 2) within the spectroscopic magnitude limit and close to the X-ray center. Indeed, the four brightest targeted central galaxies at dcen < 25″, marked in Fig. 1, were found at redshifts 1.53 ≤ z ≤ 1.56 (Fig. 3 and Table 1). The existence of extremely strong [O ii] emission lines in the galaxy spectra with IDs 3 and 4 allowed a straightforward determination of concordant redshifts at 1.554 and 1.558, respectively. The redshift determination of passive galaxies at z > 1.5 is close to the feasibility limit of FORS 2 with reasonable exposure times. However, for the BCG (ID 1) we were able to identify a weak [O ii] line and the 4000 Å-break at the limit of the wavelength coverage, to obtain a redshift of 1.556. The assigned redshift of 1.530 for the reddest galaxy (ID 2) is mainly based on a weak [O ii] line, and it therefore has a lower redshift confidence level. Based on the available spectroscopic information for XMMU J1007.4+1237, we assign a median system redshift of z = 1.555 ± 0.003. In the absence of a detailed velocity structure measurement, we treat the very red galaxy with ID 2 as a tentative member with a preliminary rest-frame velocity offset of about Δv =  −2900 km s-1.

To obtain a first assessment of the approximate ongoing star-formation activity in the member galaxies of XMMU J1007.4+1237, we measured the [O ii] equivalent widths and applied the [O ii]-SF relation of Kennicutt (1992), which provides first-order SF rates corrected for an average dust extinction in (local) star-forming systems (see Table 1). This first estimate yields SF rates for the blue galaxies with IDs 3 and 4 of about 75 and 31 M⊙ yr-1, and roughly 11 and 5 M⊙ yr-1 for the red members with IDs 1 and 2, respectively. However, these crude values could be significantly underestimated given the large uncertainties related to the dust extinction properties of high-z galaxies (e.g. Calzetti et al. 2000; Kewley et al. 2004; Pierini et al. 2005) and our limited photometric coverage.

The cluster center of XMMU J1007.4+1237 contains a 1.4 GHz radio source with a flux of 2.1 ± 0.4 mJy identified by the NVSS survey (Condon et al. 1998). This radio source, NVSS J100721+123749, is unresolved at the resolution level of the survey (45″). The brightest object within the positional radio source uncertainty of 8″ is the BCG at a distance of 4″, which is hence the most probable optical counterpart. In this case, the measured flux translates into a total radio power of P1440   MHz ≃ 3 × 1025 W Hz-1 at the cluster redshift under the assumption of a typical spectral index in the common range [−1,  −0.7] (e.g. Miley & De Breuck 2008).

3. Discussion

In the local Universe, the likely association of the BCG with a powerful radio source would classify XMMU J1007.4+1237 as a probable cool core cluster (CCC) since the radio power scales inversely with the central cooling time (Mittal et al. 2009). Additional support for a possible CCC scenario arises from the BCG location in close proximity to the center of the galaxy distribution and the extended X-ray emission, and in particular the very peaked X-ray surface brightness (Sanderson et al. 2009). However, at a lookback time of 9.4 Gyr these CCC correlations are not yet established and the central radio source might still be associated with an X-ray AGN (Hickox et al. 2009), hence point-like central X-ray emission.

Extended, non-thermal X-ray lobes associated with high-z radio galaxies have been reported for a number of z > 1 systems (e.g. Fabian et al. 2003; Erlund et al. 2008; Fabian et al. 2009; Finoguenov et al. 2010). These sources with an inverse Compton (IC) origin of their diffuse emission can in principle contribute to the measured flux or even mimic the extended thermal ICM emission characteristic for well developed clusters. A contribution of IC X-ray emission to that of XMMU J1007.4+1237 cannot presently be ruled out or constrained as this would require currently unavailable high-resolution radio observations plus very deep Chandra and XMM-Newton data. However, the available limited X-ray spectral information on XMMU J1007.4+1237 is consistent with a soft thermal spectrum and the observed overall morphological elongation is expected at this redshift by structure formation simulations (e.g. Springel et al. 2001; Boylan-Kolchin et al. 2009). Moreover, a very compact central core of the surface brightness distribution eases the X-ray selection and may be the signature of an evolved high-z system (Santos et al. 2008; Rosati et al. 2009).

With the thermal ICM assumption and the derived X-ray-luminosity-based total mass estimate of M200est≃2×1014 M⊙\hbox{$M^{\mathrm{est}}_{200}\simeq2 \times10^{14}\,M_{\sun}$}, XMMU J1007.4+1237 is currently the most massive known bona fide galaxy cluster in the “redshift desert”. We note that the goodness of the mass estimate at this distance depends more critically on the exact redshift evolution of the L − M scaling relation than the accurate X-ray luminosity, as long as the potential non-thermal contribution is sub-dominant. The applied L − M relation of Fassbender et al. (2010) assumes a slower redshift evolution than self-similar model predictions (i.e. higher mass for a given LX), fully consistent with the latest data compilation of Reichert et al. (in prep.) and recent simulations including preheating (Stanek et al. 2010).

Even though the spatial density peak of red galaxies (Fig. 2) coincides with the detected X-ray emission, the color-magnitude diagram (Fig. 3, top) markedly differs from lower-z cluster galaxy populations. While the z − H colors of the BCG and the second ranked galaxy (red squares in Fig. 3) are consistent with SSP model predictions for a high stellar formation redshift (zf ≃ 5), the total H-band magnitude of the BCG (H ∗  −0.8) with respect to the expected characteristic luminosity H* at this redshift is more than one magnitude fainter than typical counterparts at z < 1 (e.g. Smith et al. 2010), indicating that a significant fraction of the stellar BCG mass has not yet been assembled.

The third ranked, super-L ∗  galaxy (H ∗  −0.3) (left blue square and ID 3 in Fig. 3), located at a projected cluster-centric distance of only 100 kpc, is found to be a blue starburst galaxy. A second one (right blue square, ID 4) was identified at a fainter magnitude (H ∗  +0.4) at rproj ≃ 170 kpc. At this intermediate magnitude range around H ∗  ≃ 20 mag, the CMD is almost devoid of objects close to the expected color of red-sequence galaxies (red dashed line), in stark contrast to studies of massive early-type galaxies in clusters at z < 1.4 (e.g. Lidman et al. 2008; Mei et al. 2009; Lerchster et al. 2010). Hence, the cluster red-sequence in this z = 1.555 system does not yet seem to be fully in place, as some of the brightest central cluster member galaxies are still caught at an epoch of strong starburst activity.

4. Summary and conclusions

We summarize our findings as follows (see also Table 2):

  • 1.

    We have presented first results from our study on the newlydiscovered bona fide galaxy clusterXMMU J1007.4+1237 at z = 1.555, which was blindly selected as a serendipitous extended X-ray source within the XMM-Newton Distant Cluster Project (XDCP) with a subsequent identification of a coincident red galaxy population and a central BCG.

  • 2.

    On the basis of a measured bolometric X-ray luminosity of 2.1 × 1044 erg s-1 ( ± 20%), we estimated a total system mass of M200est~2×1014 M⊙\hbox{$M^{\mathrm{est}}_{200}\sim2\times10^{14}\,M_{\sun}$}, making XMMU J1007.4+1237 the most massive currently known confirmed cluster in the “redshift desert” at z > 1.5.

  • 3.

    The cluster redshift could be established based on two central starburst galaxies and the BCG within Δz = 0.004 using deep optical VLT/FORS 2 spectroscopy. A fourth associated, very red galaxy was confirmed at a rest-frame velocity offset of about  − 2900 km s-1.

  • 4.

    The central brightest cluster galaxy is located in close proximity to the X-ray peak and the maximum density of very red galaxies, and is most likely the origin of the system’s strong 1.4 GHz radio emission with P1440   MHz ≃ 3 × 1025 W Hz-1. While the BCG color is consistent with a large stellar formation redshift (zf ≃ 5), the total H-band luminosity is found to be more than one magnitude fainter than typical lower-z systems.

  • 5.

    The cluster red-sequence of XMMU J1007.4+1237 does not appear to be not fully established yet. In particular, the luminosity range close to the characteristic magnitude H* seems to be deficient in red galaxies. Consistently, the third-ranked member galaxy close to the cluster center was still caught at an epoch of strong starburst activity.

At a lookback time of 9.4 Gyr, we seem to have reached the cosmic epoch where strong evolutionary effects are still shaping the massive end of the cluster galaxy population. Upcoming deep multiwavelength observations of XMMU J1007.4+1237 promise to establish a more precise picture of the cluster constituents at this important redshift.

Online material

Table 1

Spectroscopic cluster members of XMMU J1007.4+1237.

Table 2

Properties of the galaxy cluster XMMU J1007.4+1237.

thumbnail Fig. 2

H-band view of the 3.8′ ×  3.3′ cluster environment. Red contours indicate the log-spaced projected densities of very red galaxies with colors 3 ≤ z − H ≤ 4 (see Fig. 3) spanning the levels 2–16 arcmin-2, Chandra X-ray surface brightness contours (0.7–2 keV, log-spaced) are shown in blue in the range 0.014–0.082 counts arcsec-2. 30″ (dashed) and 60″ (solid) radii around the X-ray center are marked by circles.

1

Corresponding to an extent parameter EXT ML=8.2 of the maximum likelihood fitting task emldetect, which uses a single β-model with a fixed β = 0.667 as a model profile for the source extent determination.

2

Using z for the blue channel, z + H for green, and H for red.

Acknowledgments

We acknowledge the excellent support provided by Calar Alto and VLT staff in carrying out the service observations. This research was supported by the DFG cluster of excellence Origin and Structure of the Universe (www.universe-cluster.de), by the DFG under grants Schw536/24-1, Schw 536/24-2, BO 702/16-3, and the German DLR under grant 50 QR 0802. R.F. acknowledges the hospitality of the Department of Astronomy and Astrophysics at Pontificia Universidad Católica de Chile. H.Q. thanks the FONDAP Centro de Astrofisica for partial support. The XMM-Newton project is 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 NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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All Tables

Table 1

Spectroscopic cluster members of XMMU J1007.4+1237.

In the text
Table 2

Properties of the galaxy cluster XMMU J1007.4+1237.

In the text

All Figures

thumbnail Fig. 1

Optical/NIR appearance of the galaxy cluster XMMU J1007.4+1237 at z = 1.555 with XMM-Newton X-ray contours overlaid in cyan in the logarithmically spaced range [0.014, 0.074] counts arcsec-2 (0.35–2.4 keV). Left: z + H-band color composite (2.3′ ×  2.3′) centered on the cluster. Spectroscopic cluster members are marked by white circles, 30″ (dashed) and 60″ (solid) radii around the X-ray center are indicated in yellow. Right: H-band zoom on the cluster core region. Red (IDs 1 and 2) and blue starburst (IDs 3 and 4) cluster members are marked by small circles, spectroscopically confirmed foreground galaxies are indicated by green crosses, and the outer dashed circle represents the 30″ radius.
In the text
thumbnail Fig. 3

Top: z − H vs. H CMD of the cluster field. Galaxies with projected cluster-centric distances of  < 30″ (30–60″) are displayed in red (green). Secure spectroscopic cluster members are marked by open squares, foreground objects are crossed-out. The vertical blue line indicates the apparent H magnitude of an L ∗  galaxy at z = 1.56, the black line shows the H-band magnitude limit. Two solar metallicity SSP models for formation redshifts of zf = 5 (red dashed line) and zf = 3 (blue dashed) are overplotted. Bottom: VLT/FORS 2 spectra of three secure cluster members smoothed with a 7 pixel boxcar filter, associated IDs (1, 3, 4) refer to Table 1. Redshifted spectral features are indicated together with an overlaid LRG template spectrum in blue (ID 1), sky emission lines (bottom) and telluric absorption (top) are shown in red.
In the text
thumbnail Fig. 2

H-band view of the 3.8′ ×  3.3′ cluster environment. Red contours indicate the log-spaced projected densities of very red galaxies with colors 3 ≤ z − H ≤ 4 (see Fig. 3) spanning the levels 2–16 arcmin-2, Chandra X-ray surface brightness contours (0.7–2 keV, log-spaced) are shown in blue in the range 0.014–0.082 counts arcsec-2. 30″ (dashed) and 60″ (solid) radii around the X-ray center are marked by circles.

In the text

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