R. Fassbender

Discovery of an X-Ray-luminous Galaxy Cluster at z = 1.4

We report the discovery of a massive, X-ray-luminous cluster of galaxies at z = 1.393, the most distant X-ray-selected cluster found to date. XMMU J2235.3-2557 was serendipitously detected as an extended X-ray source in an archival XMM-Newton observation of NGC 7314. VLT FORS2 R- and z-band snapshot imaging reveals an overdensity of red galaxies in both angular and color spaces. The galaxy enhancement is coincident in the sky with the X-ray emission; the cluster red sequence at R-z 2.1 identifies it as a high-redshift candidate. Subsequent FORS2 multiobject spectroscopy unambiguously confirms the presence of a massive cluster based on 12 concordant redshifts in the interval 1.38 1 cluster found with XMM-Newton, the relative ease and efficiency of discovery demonstrates that it should be possible to build large samples of z > 1 clusters through the joint use of X-ray and large ground-based telescopes.

Multi-wavelength study of XMMU J2235.3-2557: the most massive galaxy cluster at z > 1

Context. The galaxy cluster XMMU J2235.3−2557 (hereafter XMM2235), spectroscopically confirmed at z = 1.39, is one of the most distant X-ray selected galaxy clusters. It has been at the center of a multi-wavelength observing campaign with ground and space facilities. Aims. We characterize the galaxy populations of passive members, the thermodynamical properties and metal abundance of the hot gas, and the total mass of the system using imaging data with HST/ACS (i775 and z850 bands) and VLT/ISAAC (J and KS bands), extensive spectroscopic data obtained with VLT/FORS2, and deep (196 ks) Chandra observations. Methods. Chandra data allow temperature and metallicity to be measured with good accuracy and the X-ray surface brightness profile to be traced out to 1 � (or 500 kpc), thus allowing the mass to be reliably estimated. Out of a total sample of 34 spectroscopically confirmed cluster members, we selected 16 passive galaxies (without detectable [OII]) within the central 2 � (or 1 Mpc) with ACS coverage, and inferred star formation histories for subsamples of galaxies inside and outside the core by modeling their spectrophotometric data with spectral synthesis models. Results. Chandra data show a regular elongated morphology, closely resembling the distribution of core galaxies, with a significant cool core. We measure a global X-ray temperature of kT = 8.6 +1.3 −1.2 keV (68% confidence), which we find to be robust against several systematics involved in the X-ray spectral analysis. By detecting the rest frame 6.7 keV Iron K line in the Chandra spectrum, we measure a metallicity Z = 0.26 +0.20 −0.16 Z� . In the likely hypothesis of hydrostatic equilibrium, we obtain a total mass of Mtot( 1, with a baryonic content, both its galaxy population and intracluster gas, in a significantly advanced evolutionary stage at 1/ 3o f the current age of the Universe.

Cluster galaxies in XMMU J2235-2557: galaxy population properties in most massive environments at z ∼1.4

We present a multi-wavelength study of galaxy populations in the core of the massive, X-ray luminous cluster XMMU J2235 at z=1.39, based on high quality VLT and HST photometry at optical and near-infrared wavelengths. We derive luminosity functions in the z, H, and Ks bands, approximately corresponding to restframe U, R and z band. These show a faint-end slope consistent with being flat, and a character istic magnitude Mclose to passive evolution predictions of Mof local massive clusters, with a formation redshift z> 2. The color-magnitude and color-mass diagrams show evidence of a tight red sequence (intrinsic scatter . 0: 08) of massive galaxies already in place, with overall old stellar populations and g enerally early-type morphology. Beside the red colors, these massive (> 6� 10 10 M�) galaxies typically show early-type spectral features, an d rest-frame far-UV emission consistent with very low star formation rates (SFR< 0: 2M�/yr). Star forming spectroscopic members, with SFR of up to� 100M�/yr, are all located at clustercentric distances &250kpc, with the central cluster region already appearing effectively quenched. Most part of the cluster galaxies more massive than 6� 10 10 Mwithin the studied area do not appear to host significant levels of st ar formation. The high-mass end galaxy populations in the core of this cluster appear to be in a very advanced evolutionary stage, not only in terms of formation of the stellar populations, but also of the asse mbly of the stellar mass. The high-mass end of the galaxy stellar mass function is essentially already in place. The stellar mass f raction estimated within r500 (�1%, Kroupa IMF) is already similar to that of local massive clusters. On the other hand, surface brightness distribution modeling of the massive red sequence galaxies may suggest that their size is often smaller than expected based on the local stellar mass vs size relation. An evolution of the stellar mass vs size relation m ight imply that, in spite of the overall early assembly of these sources , their evolution is not complete, and processes like minor ( and likely dry) merging might still shape the structural properties of thes e objects to resemble those of their local counterparts, wit hout substantially affecting their stellar mass or host stellar populations. None theless, a definite conclusion on the actual relevance of siz e evolution for the studied early-type sample is precluded by possible systematics and biases.

Discovery of a massive X-ray luminous galaxy cluster at z = 1.579

We report on the discovery of a very distant galaxy cluster serendipitously detected in the archive of the XMM-Newton mission, within the scope of the XMM-Newton Distant Cluster Project (XDCP). XMMUJ0044.0-2033 was detected at a high significance level (5σ) as a compact, but significantly extended source in the X-ray data, with a soft-band flux f (r 1.6.

The X-ray luminous galaxy cluster XMMU J1007.4+1237 at z = 1.56 - the dawn of starburst activity in cluster cores

Context. Observational galaxy cluster studies at z > 1.5 probe the formation of the first massive M > 10 14 M ⊙ dark matter halos, the early thermal history of the hot ICM, and the emergence of the red-sequence population of quenched early-type galaxies. Aims. We present first results for the newly discovered X-ray luminous galaxy cluster XMMUJ1007.4+1237 at z = 1.555, detected and confirmed by the XMM-Newton Distant Cluster Project (XDCP) survey. Methods. We selected the system as a serendipitous weak extended X-ray source in XMM-Newton archival data and followed it up with two-band near-infrared imaging and deep optical spectroscopy. Results. We can establish XMMU J 1007.4+1237 as a spectroscopically confirmed, massive, bona fide galaxy cluster with a bolometric X-ray luminosity of L bol X,500 ≃ (2.1 ± 0.4) x 10 44 erg/s, a red galaxy population centered on the X-ray emission, and a central radio-loud brightest cluster galaxy. However, we see evidence for the first time that the massive end of the galaxy population and the cluster red-sequence are not yet fully in place. In particular, we find ongoing starburst activity for the third ranked galaxy close to the center and another slightly fainter object. Conclusions. At a lookback time of 9.4 Gyr, the cluster galaxy population appears to be caught in an important evolutionary phase, prior to full star-formation quenching and mass assembly in the core region. X-ray selection techniques are an efficient means of identifying and probing the most distant clusters without any prior assumptions about their galaxy content.

Observational constraints on the redshift evolution of X-ray scaling relations of galaxy clusters out to z ~ 1.5

Context. A precise understanding of the relations between observable X-ray properties of galaxy clusters and cluster mass is a vital part of the application of X-ray galaxy cluster surveys to te st cosmological models. An understanding of how these relations evolve with redshift is just emerging from a number of observational data sets. Aims. The current literature provides a diverse and inhomogeneous picture of scaling relation evolution. We attempt to trans form these results and the data on recently discovered distant cl usters into an updated and consistent framework, and provide an overall view of scaling relation evolution from the combined data sets. Methods. We study in particular the most important scaling relations connecting X-ray luminosity, temperature, and cluster mass (M‐ T, LX‐T, and M‐LX) combining 14 published data sets supplemented with recently published data of distant clusters and new results from follow-up observations of the XMM-Newton Distant Cluster Project (XDCP) that adds new leverage to effi ciently constrain the scaling relations at high redshift. Results. We find that the evolution of the mass-temperature relation i s consistent with the self-similar evolution prediction, w hile the evolution of X-ray luminosity for a given temperature and mass for a given X-ray luminosity is slower than predicted by simple self-similar models. Our best fit results for the evolution f actor E(z) � are �=−1.04± 0.07 for the M‐T relation, �=−0.23 +0.12 −0.62 for the L-T relation, and �=−0.93 +0.62 −0.12 for the M‐LX relation. We also explore the influence of selection e ffects on scaling relations and find that selection biases are the most likely reason for apparen t inconsistencies between different published data sets. Conclusions. The new results provide the currently most robust calibration of high-redshift cluster mass estimates based on X-ray luminosity and temperature and help us to improve the prediction of the number of clusters to be found in future galaxy cluster X-ray surveys, such as eROSITA. The comparison of evolution results with hydrodynamical cosmological simulations suggests that early preheating of the intracluster medium (ICM) provides the most suitable scenario to explain the observed evolution.

Multiwavelength observations of a rich galaxy cluster at z ∼ 1: The HST/ACS colour - Magnitude diagram

Context. XMMU J1229+0151 is a rich galaxy cluster with redshift z = 0.975 that was serendipitously detected in X-rays within the scope of the XMM-Newton Distant Cluster Project. Both HST/ACS observations in the i775 and z850 passbands and VLT/FORS2 spectroscopy were obtained, in addition to follow-up Near-Infrared (NIR) imaging in the J -a ndKs-bands with NTT/SOFI. Aims. We investigate the photometric, structural, and spectral properties of the early-type galaxies in the high-redshift cluster XMMU J1229+0151. Methods. Source detection and aperture photometry are performed in the optical and NIR imaging. Galaxy morphology is inspected visually and by means of Sersic profile fitting to the 21 spectroscopically confirmed cluster members in the ACS field of view. The i775 − z850 colour−magnitude relation (CMR) is derived with a method based on galaxy magnitudes obtained by fitting the surface brightness of the galaxies with Sersic models. Stellar masses and formation ages of the cluster galaxies are derived by fitting the observed spectral energy distributions (SED) with models developed by Bruzual & Charlot. Star-formation histories of the early-type galaxies are constrained by analysing the stacked spectrophotometric data. Results. The structural Sersic index n obtained by model fitting agrees with the visual morphological classification of the confirmed members, indicating a clear predominance of elliptical galaxies (15/21). The i775 −z850 colour−magnitude relation of the spectroscopic members shows a very tight red-sequence with a zero point of 0.86 ± 0.04 mag, and intrinsic scatter equal to 0.039 mag. The CMR obtained with the galaxy models has similar parameters. By fitting both the spectra and SED of the early-type population, we obtain a star-formation-weighted age of 4.3 Gyr for a median galaxy stellar-mass of 7.4 × 10 10 M� . Instead of identifying a brightest cluster galaxy (BCG) unambiguously, we find three bright galaxies with a similar z850 magnitude, which are, in addition, the most massive cluster members, with ∼2 × 10 11 M� . Our results strengthen the current evidence of a lack of significant evolution in both the scatter and slope of the red-sequence out to z ∼ 1.

A Large-Scale Galaxy Structure At Z=2.02 Associated With The Radio Galaxy MRC 0156-252

We present the spectroscopic confirmation of a structure of galaxies surrounding the radio galaxy MRC 0156-252 at z = 2.02. The structure was initially discovered as an overdensity of both near-infrared selected z > 1.6 and mid-infrared selected z > 1.2 galaxy candidates. We used the VLT/FORS2 multi-object spectrograph to target ∼80 high-redshift galaxy candidates, and obtain robust spectroscopic redshifts for more than half the targets. The majority of the confirmed sources are star-forming galaxies at z > 1.5. In addition to the radio galaxy, two of its close-by companions (<6 �� ) also show AGN signatures. Ten sources, including the radio galaxy, lie within | z − 2.020 | <0.015 (i.e., velocity offsets <1500 km s −1 ) and within projected 2 Mpc comoving of the radio galaxy. Additional evidence suggests not only that the galaxy structure associated with MRC 0156-252 is a forming galaxy cluster but also that this structure is most probably embedded in a larger-scale structure.

The massive galaxy cluster XMMU J1230.3+1339 at z ∼ 1: colour–magnitude relation, Butcher–Oemler effect, X‐ray and weak lensing mass estimates★

We present results from the multiwavelength study of XMMU J1230.3+1339 at z ∼ 1. We analyse deep multiband wide-field images from the Large Binocular Telescope (LBT), multi-object spectroscopy observations from VLT, as well as space-based serendipitous observations, from the GALEX and Chandra X-ray observatories. We apply a Bayesian photometric redshift code to derive the redshifts using the far-UV (FUV), near-UV (NUV) and the deep U, B, V, r, i, z data. We make further use of spectroscopic data from FORS2 to calibrate our photometric redshifts, and investigate the photometric and spectral properties of the early-type galaxies. We achieve an accuracy of Δz/(1 + z) = 0.07 (0.04) and the fraction of catastrophic outliers is η = 13 (0) per cent, when using all (secure) spectroscopic data, respectively. The i ― z against z colour-magnitude relation of the photo-z members shows a tight red sequence with a zero-point of 0.935 mag, and slope equal to -0.027. We observe evidence for a truncation at the faint end of the red-cluster-sequence and the Butcher-Oemler effect, finding a fraction of blue galaxies f b ≈ 0.5. Further, we conduct a weak lensing analysis of the deep 26 x 26 arcmin r-band LBC image. The observed shear is fitted with a Single-Isothermal-Sphere and a Navarro-Frenk-White model to obtain the velocity dispersion and the model parameters, respectively. Our best-fitting values are, for the velocity dispersion σ SIS = 1308 ± 284 km s ―1 , concentration parameter c = 4.0 +4 ―2 and scale radius r s = 345 +50 ―57 kpc. From a 38 ks Chandra X-ray observation we obtain an independent estimate of the cluster mass. In addition, we create a signal-to-noise ratio (S/N) map for the detection of the matter mass distribution of the cluster using the mass-aperture technique. We find excellent agreement of the mass concentration identified with weak lensing and the X-ray surface brightness. Combining our mass estimates from the kinematic, X-ray and weak lensing analyses we obtain a total cluster mass of M tot 200 = (4.56 ± 2.3) x 10 14 M ⊙ . This study demonstrates the feasibility of ground-based weak lensing measurements of galaxy clusters up to z ∼ 1.

Discovery of the X-ray selected galaxy cluster XMMU J0338.8+0021 at z = 1.49 - Indications of a young system with a brightest galaxy in formation

We report the discovery of a galaxy cluster at z = 1.490 originally selected as an extended X-ray source in the XMM-Newton Distant Cluster Project. Further observations carried out with the VLT-FORS2 spectrograph allowed the spectroscopic confirmation of seven secure cluster members, providing a median system redshift of z = 1.490 ± 0.009. The color-magnitude diagram of XMMU J0338.8+0021 reveals the presence of a well-populated red sequence with z − H ≈ 3, albeit with an apparent significant scatter in color. Since we do not detect indications of any strong star formation activity in these objects, the color spread could represent the different stellar ages of the member galaxies. In addition, we found the brightest cluster galaxy in a very active dynamical state, with an interacting, merging companion located at a physical projected distance of d ≈ 20 kpc. From the X-ray luminosity, we estimate a cluster mass of M200 ∼ 1.2 × 10 14 M� . The data appear to be consistent with a scenario in which XMMU J0338.8+0021 is