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It is important to map the large-scale matter distribution in the local Universe for cosmological studies, such as the tracing of the large-scale peculiar velocity flow, the characterisation of the environment for different astronomical objects, and for precision measurements of cosmological parameters. We used X-ray luminous clusters to map this matter distribution and find that about 51% of the groups and clusters are members of superclusters which occupy only a few percent of the volume. In this paper we provide a detailed description of these large-scale structures. With a friends-to-friends algorithm, we find eight superclusters with a cluster overdensity ratio of at least two with five or more galaxy group and cluster members in the cosmic volume out to z = 0.03. The four most prominent ones are the Perseus-Pisces, the Centaurus, the Coma, and the Hercules supercluster, with lengths from about 40 to over 100 Mpc and estimated masses of 0.6 - 2.2 10^16 Msun. The largest of these structures is the Perseus-Pisces supercluster. The four smaller superclusters include the Local and the Abell 400 supercluster and two superclusters in the constellations Sagittarius and Lacerta. We provide detailed maps, member catalogues, and physical descriptions of the eight superclusters. By constructing superclusters with a range of cluster sub-samples with different lower X-ray luminosity limits, we show that the main structures are always reliably recovered., Comment: Published in Astronomy and Astrophysics, 31 pages, 28 figures

The galaxy cluster system RXCJ0225.9-4154 with the two sub-clusters A3016 and A3017 is embedded in a large-scale structure filament with signatures of filamentary accretion. A Chandra observation of this system shows the two clusters as well as a filament of X-ray emission connecting them and a galaxy group therein. The main cluster, A3017, shows indications of shocks most probably from a recent interaction with cluster components along the filament axis as well as a cold front. The filament between the two clusters is likely to be heated by the accretion shocks of the clusters and we discuss two scenarios for the origin of this X-ray filament. The paper describes many properties of the components of the cluster merger system that are used to assist the interpretation of the observed configuration., Comment: 7 pages, 11 figures

Previous studies of the galaxy and galaxy cluster distribution in the local Universe found indications for a large extension of the Local Supercluster up to a radius of 190 h_70^-1 Mpc. We are using our large and highly complete CLASSIX survey of X-ray luminous galaxy clusters detected in the ROSAT All Sky Survey to trace the matter distribution in the local Universe and to explore the size of the flattened local density structure associated with the Local Supercluster. The Local Supercluster is oriented almost perpendicular to the Galactic plane. Since Galactic extinction increases towards the Galactic plane, objects are on average more easily visible perpendicular to the plane than close to it, also producing an apparent concentration of objects along the Local Supercluster. We can correct for this bias by a careful treatment of the survey selection function. We find a significant overdensity of clusters in a flattened structure along the Supergalactic plane with a thickness of about 50 Mpc and an extent of about 100 Mpc radius. Structures at a distance larger than 100 Mpc are not correlated to the Local Supercluster any more. The matter density contrast of the local superstructure to the surroundings is about a factor of 1.3 - 2.3. Within the Supergalactic plane the matter is concentrated mostly in two superclusters, the Perseus-Pisces Chain and Hydra-Centaurus supercluster. We have shown in our earlier work that the local Universe in a region with a radius of 100 - 170 Mpc has a lower density than the cosmic mean. For this reason, the Local Supercluster is not overdense with respect to the cosmic mean density. Therefore this local superstructure will not collapse as a whole in the future, but rather fragment., Comment: Astronomy and Astrophysics in press; 1 pages, 7 figures

The Perseus-Pisces supercluster is known as one of the largest structures in the nearby Universe that has been charted by the galaxy and galaxy cluster distributions. For the latter mostly clusters from the Abell catalogue have been used. Here we take a new approach to a quantitative characterisation of the Perseus-Pisces supercluster using a statistically complete sample of X-ray luminous galaxy groups and clusters from our CLASSIX galaxy cluster redshift survey. We used a friends-of-friends technique to construct the supercluster membership. We also studied the structure of the Southern Great Wall, which merges with the Perseus-Pisces supercluster with a slightly increased friends-of-friends linking length. In this work we discuss the geometric structure of the superclusters, compare the X-ray luminosity distribution of the members with that of the surroundings, and provide an estimate of the supercluster mass. These results establish Perseus-Pisces as the largest superstructure in the Universe at redshifts z <= 0.03. With the new data this supercluster extends through the zone of avoidance, which has also been indicated by some studies of the galaxy distribution by means of HI observations. We investigated whether the shapes of the member groups and clusters in X-rays are aligned with the major axis of the supercluster. We find no evidence for a pronounced alignment, except for the ellipticities of Perseus and AWM7, which are aligned with the separation vector of the two systems and weakly with the supercluster., Comment: Astronomy and Astrophysics in press; 17 pages, 13 figures

For precision cosmological studies it is important to know the local properties of the reference point from which we observe the Universe. Particularly for the determination of the Hubble constant with low-redshift distance indicators, the values observed depend on the average matter density within the distance range covered. Here we used the spatial distribution of galaxy clusters to map the matter density distribution. The study is based on our CLASSIX galaxy cluster survey, which is highly complete and well characterised with galaxy clusters detected in X-rays. We find a local underdensity in the cluster distribution of about 30 - 60% which extends ~85 Mpc to the north and ~170 Mpc to the south. For three regions for which the galaxy density distribution has previously been studied, we find good agreement between the density distribution of clusters and galaxies. Correcting for the bias in the cluster distribution we infer an underdensity in the matter distribution of about -0.3 +- 0.15 (-0.2 +- 0.1) in a region with a radius of about 100 (~140) Mpc. Calculating the probability of finding such an underdensity theoretically in a LambdaCDM universe with concordance cosmological parameters, we find a probability characterised by sigma-values of 1.3-3.7. This indicates low probabilities, but with values of around 10% at the lower uncertainty limit, the existence of an underdensity cannot be ruled out. Inside this underdensity, the observed Hubble parameter will be larger by about 5.5 +2.1-2.8%, which explains part of the discrepancy between the locally measured value of H_0 compared to the value of H_0 inferred from the Planck observations of cosmic microwave background anisotropies. If distance indicators outside the local underdensity are included, as in many modern analyses, this effect is diluted., Comment: 11 pages, 17 figures, Astronomy and Astrophysics in press

The mass function of galaxies and clusters of galaxies can be derived observationally based on different types of observations. In this study we test if these observations can be combined to a consistent picture which is also in accord with structure formation theory. The galaxy data comprise the optical galaxy luminosity function and the gravitational lensing signature of the galaxies, while the galaxy cluster mass function is derived from the X-ray luminosity distribution of the clusters. We show the results of the comparison in the form of the mass density fraction that is contained in collapsed objects relative to the mean matter density in the Universe. The mass density fraction in groups and clusters of galaxies extrapolated to low masses agrees very well with that of the galaxies: both converge at the low mass limit to a mass fraction of about 28\% if the outer radii of the objects are taken to be $r_{200}$. Most of the matter contained in collapsed objects is found in the mass range $M_{200} \sim 10^{12} - 10^{14} h^{-1}_{70} M_\odot$, while a larger amount of the cosmic matter resides outside of objects with radius $r_{200}$., Comment: To appear in MNRAS

Supermassive black hole binaries may be detectable by an upcoming suite of gravitational wave experiments. Their binary nature can also be revealed by radio jets via a short-period precession driven by the orbital motion as well as the geodetic precession at typically longer periods. We have investigated Karl G. Jansky Very Large Array (VLA) and MERLIN radio maps of powerful jet sources for morphological evidence of geodetic precession. For perhaps the best studied source, Cygnus A, we find strong evidence for geodetic precession. Projection effects can enhance precession features, for which we find indications in strongly projected sources. For a complete sample of 33 3CR radio sources we find strong evidence for jet precession in 24 cases (73 per cent). The morphology of the radio maps suggests that the precession periods are of the order of 10^6 - 10^7 yr. We consider different explanations for the morphological features and conclude that geodetic precession is the best explanation. The frequently observed gradual jet angle changes in samples of powerful blazars can be explained by orbital motion. Both observations can be explained simultaneously by postulating that a high fraction of powerful radio sources have sub-parsec supermassive black hole binaries. We consider complementary evidence and discuss if any jetted supermassive black hole with some indication of precession could be detected as individual gravitational wave source in the near future. This appears unlikely, with the possible exception of M87., Comment: 24 pages, 36 figures, accepted by MNRAS

We study the evolution of substructure in the mass distribution with mass, redshift and radius in a sample of simulated galaxy clusters. The sample, containing $1226$ objects, spans the mass range $M_{200} = 10^{14} - 1.74 \times 10^{15} \ {\rm M_{\odot}} \ h^{-1}$ in six redshift bins from $z=0$ to $z=1.179$. We consider three different diagnostics: 1) subhalos identified with SUBFIND; 2) overdense regions localized by dividing the cluster into octants; 3) offset between the potential minimum and the center of mass. The octant analysis is a new method that we introduce in this work. We find that none of the diagnostics indicate a correlation between the mass of the cluster and the fraction of substructures. On the other hand, all the diagnostics suggest an evolution of substructures with redshift. For SUBFIND halos, the mass fraction is constant with redshift at $R_{\mathrm{vir}}$, but shows a mild evolution at $R_{200}$ and $R_{500}$. Also, the fraction of clusters with at least a subhalo more massive than one thirtieth of the total mass is less than $20 \%$. Our new method based on the octants returns a mass fraction in substructures which has a strong evolution with redshift at all radii. The offsets also evolve strongly with redshift. We also find a strong correlation for individual clusters between the offset and the fraction of substructures identified with the octant analysis. Our work puts strong constraints on the amount of substructures we expect to find in galaxy clusters and on their evolution with redshift., Comment: 12 pages, 10 figures, 8 tables, accepted for publication in MNRAS

We present analysis results for a nearby galaxy cluster Abell 1631 at $z~=~0.046$ using the X-ray observatory Suzaku. This cluster is categorized as a low X-ray surface brightness cluster. To study the dynamical state of the cluster, we conduct four-pointed Suzaku observations and investigate physical properties of the Mpc-scale hot gas associated with the A1631 cluster for the first time. Unlike relaxed clusters, the X-ray image shows no strong peak at the center and an irregular morphology. We perform spectral analysis and investigate the radial profiles of the gas temperature, density, and entropy out to approximately 1.5~Mpc in the east, north, west, and south directions by combining with the XMM-Newton data archive. The measured gas density in the central region is relatively low (${\rm a~few} \times~10^{-4}~{\rm cm^{-3}}$) at the given temperature ($\sim2.9~{\rm keV}$) compared with X-ray-selected clusters. The entropy profile and value within the central region ($r<0.1~r_{200}$) are found to be flatter and higher ($\gtrsim400~ {\rm keV~cm}^2$). The observed bolometric luminosity is approximately three times lower than that expected from the luminosity-temperature relation in previous studies for relaxed clusters. These features are also observed in another low surface brightness cluster, Abell 76. The spatial distributions of galaxies and the hot gas appear to be different. The X-ray luminosity is relatively lower than that expected from the velocity dispersion. A post-merger scenario may explain the observed results., Comment: 18 pages, 4 figures, accepted for publication in PASJ

We apply a friends-of-friends algorithm to the HectoMAP redshift survey and cross-identify associated X-ray emission in the ROSAT All-Sky Survey data (RASS). The resulting flux limited catalog of X-ray cluster survey is complete to a limiting flux of $\sim3 \times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ and includes 15 clusters (7 newly discovered) with redshift $z \leq 0.4$. HectoMAP is a dense survey ($\sim1200$ galaxies deg$^{-2}$) that provides $\sim50$ members (median) in each X-ray cluster. We provide redshifts for the 1036 cluster members. Subaru/Hyper Suprime-Cam imaging covers three of the X-ray systems and confirms that they are impressive clusters. The HectoMAP X-ray clusters have an $L_{X} - {\sigma}_{cl}$ scaling relation similar to that of known massive X-ray clusters. The HectoMAP X-ray cluster sample predicts $\sim 12000 \pm3000$ detectable X-ray clusters in the RASS to the limiting flux, comparable with previous estimates., Comment: submitted to ApJ, a revised version in response to the referee's comments, 18 pages, 12 figures, 4 tables, Table 3 will be available when the paper is accepted

We present first strong observational evidence that the X-ray cool-core bias or the apparent bias in the abundance of relaxed clusters is absent in our REFLEX volume-limited sample (ReVols). We show that these previously observed biases are due to the survey selection method such as for an flux-limited survey, and are not due to the inherent nature of X-ray selection. We also find that the X-ray luminosity distributions of clusters for the relaxed and for the disturbed clusters are distinct and a displacement of approximately 60 per cent is required to match two distributions. Our results suggest that to achieve more precise scaling relation one may need to take the morphology of clusters and their fractional abundance into account., Comment: A&A, 606, L4, 4 pages, 3 figures

The mass function of galaxy clusters is a sensitive tracer of the gravitational evolution of the cosmic large-scale structure and serves as an important census of the fraction of matter bound in large structures. We obtain the mass function by fitting the observed cluster X-ray luminosity distribution from the REFLEX galaxy cluster survey to models of cosmological structure formation. We marginalise over uncertainties in the cosmological parameters as well as those of the relevant galaxy cluster scaling relations. The mass function is determined with an uncertainty less than 10% in the mass range 3 x 10^12 to 5 x 10^14 M$_\odot$. For the cumulative mass function we find a slope at the low mass end consistent with a value of -1, while the mass rich end cut-off is milder than a Schechter function with an exponential term exp($- M^\delta$) with $\delta$ smaller than 1. Changing the Hubble parameter in the range $H_0 = 67 - 73 km s^-1 Mpc^{-1}$ or allowing the total neutrino mass to have a value between 0 - 0.4 eV causes variations less than the uncertainties. We estimate the fraction of mass locked up in galaxy clusters: about 4.4% of the matter in the Universe is bound in clusters (inside $r_200$) with a mass larger than 10^14 M$_\odot$ and 14% to clusters and groups with a mass larger than 10^13 M$_\odot$ at the present Universe. We also discuss the evolution of the galaxy cluster population with redshift. Our results imply that there is hardly any clusters with a mass > 10^15 M$_\odot$ above a redshift of z = 1., Comment: 7 pages, 8 figures, submitted to Astronomy and Astrophysics

As the largest, clearly defined building blocks of our Universe, galaxy clusters are interesting astrophysical laboratories and important probes for cosmology. X-ray surveys for galaxy clusters provide one of the best ways to characterise the population of galaxy clusters. We provide a description of the construction of the NORAS II galaxy cluster survey based on X-ray data from the northern part of the ROSAT All-Sky Survey. NORAS II extends the NORAS survey down to a flux limit of 1.8 x 10^(-12) erg s^-1 cm^-2 (0.1 - 2.4 keV) increasing the sample size by about a factor of two. The NORAS II cluster survey now reaches the same quality and depth of its counterpart, the Southern REFLEX II survey, allowing us to combine the two complementary surveys. The paper provides information on the determination of the cluster X-ray parameters, the identification process of the X-ray sources, the statistics of the survey, and the construction of the survey selection function, which we provide in numerical format. Currently NORAS II contains 860 clusters with a median redshift of z = 0.102. We provide a number of statistical functions including the logN-logS and the X-ray luminosity function and compare these to the results from the complementary REFLEX II survey. Using the NORAS II sample to constrain the cosmological parameters, sigma_8 and Omega_m, yields results perfectly consistent with those of REFLEX II. Overall, the results show that the two hemisphere samples, NORAS II and REFLEX II, can be combined without problems to an all-sky sample, just excluding the Zone-of-Avoidance., Comment: 14 pages, 13 figures

We derive and compare the fractions of cool-core clusters in the {\em Planck} Early Sunyaev-Zel'dovich sample of 164 clusters with $z \leq 0.35$ and in a flux-limited X-ray sample of 100 clusters with $z \leq 0.30$, using {\em Chandra} observations. We use four metrics to identify cool-core clusters: 1) the concentration parameter: the ratio of the integrated emissivity profile within 0.15 $r_{500}$ to that within $r_{500}$, and 2) the ratio of the integrated emissivity profile within 40 kpc to that within 400 kpc, 3) the cuspiness of the gas density profile: the negative of the logarithmic derivative of the gas density with respect to the radius, measured at 0.04 $r_{500}$, and 4) the central gas density, measured at 0.01 $r_{500}$. We find that the sample of X-ray selected clusters, as characterized by each of these metrics, contains a significantly larger fraction of cool-core clusters compared to the sample of SZ selected clusters (44$\pm$7\% vs. 28$\pm$4\% using the concentration parameter in the 0.15--1.0 $r_{500}$ range, 61$\pm$8\% vs. 36$\pm$5\% using the concentration parameter in the 40--400 kpc range, 64$\pm$8\% vs. 38$\pm$5\% using the cuspiness, and 53$\pm$7\% vs. 39$\pm$5\% using the central gas density). Qualitatively, cool-core clusters are more X-ray luminous at fixed mass. Hence, our X-ray flux-limited sample, compared to the approximately mass-limited SZ sample, is over-represented with cool-core clusters. We describe a simple quantitative model that uses the excess luminosity of cool-core clusters compared to non-cool-core clusters at fixed mass to successfully predict the observed fraction of cool-core clusters in X-ray selected samples., Comment: 18 pages, 6 figures, 5 tables. Accepted for publication in ApJ

We report the discovery of a potentially major supercluster that extends across the Galactic Plane in the constellation of Vela, at a mean recessional velocity of ~18,000 km/s. Recent multi-object spectroscopic observations of this Vela Supercluster (VSCL), using AAOmega+2dF and the Southern African Large Telescope, confirm an extended galaxy overdensity in the Zone of Avoidance (ZOA) located where residual bulk flows predict a considerable mass excess. We present a preliminary analysis of ~4,500 new spectroscopic galaxy redshifts obtained in the ZOA centred on the Vela region (l=272.5+-20 deg, b=0+-10 deg). The presently sparsely-sampled dataset traces an overdensity that covers 25 deg in Galactic longitude on either side of the Plane, suggesting an extent of 25 deg x 20 deg, corresponding to ~115 x 90 $h_{70}$ Mpc at the supercluster redshift. In redshift space, the overdensity appears to consist of two merging wall-like structures, interspersed with clusters and groups. Both the velocity histogram and the morphology of the multi-branching wall structure are consistent with a supercluster classification. $K_s^o$ galaxy counts show an enhancement of ~1.2 over the survey area for galaxies brighter than $M_K^*$ at the VSCL distance, and a galaxy overdensity of $\delta=0.50\rm{-}0.77$ within a photometric redshift shell around the VSCL, when compared to various Two-Micron All-Sky Survey samples. Taking account of selection effects, the VSCL is estimated to contribute $v_\rm{LG} \gtrsim 50$ km/s to the motion of the Local Group., Comment: Matches the final version published in MNRAS Letters. Updated author list, spelling, references

To search for a signature of an intracluster magnetic field, we compare measurements of Faraday rotation of polarised extragalactic radio sources in the line of sight of galaxy clusters with those outside. We correlated a catalogue of 1383 rotation measures (RM) of extragalactic polarised radio sources with X-ray luminous galaxy clusters from the CLASSIX survey (combining REFLEX II and NORAS II). We compared the RM in the line of sight of clusters within their projected radii of r_500 with those outside and found a significant excess of the dispersion of the RM in the cluster regions. Since the observed RM is the result of Faraday rotation in several presumably uncorrelated magnetised cells of the intracluster medium, the observations correspond to quantities averaged over several magnetic field directions and strengths. Therefore the interesting quantity is the standard deviation of the RM for an ensemble of clusters. We found a standard deviation of the RM inside r_500 of about 120 +- 21 rad m^-2. This compares to about 56 +- 8 rad m^-2 outside. We show that the most X-ray luminous and thus most massive clusters contribute most to the observed excess RM. Modelling the electron density distribution in the intracluster medium with a self-similar model, we found that the dispersion of the RM increases with the column density, and we deduce a magnetic field value of about 2 - 6 (l/10kpc)^-1/2 microG assuming a constant magnetic field strength, where l is the size of the coherently magnetised intracluster medium cells. This magnetic field energy density amounts to a few percent of the average thermal energy density in clusters. When we assumed the magnetic energy density to be a constant fraction of the thermal energy density, we deduced a slightly lower value for this fraction of 3 - 10 (l/10kpc)^-1/2 per mille., Comment: 7 pages, 6 figures, in press, Astronomy and Astrophysics, 2016

The high precision measurements of the cosmic microwave background by the Planck survey yielded tight constraints on cosmological parameters and the statistics of the density fluctuations at the time of recombination. This provides the means for a critical study of structure formation in the Universe by comparing the microwave background results with present epoch measurements of the cosmic large-scale structure. It can reveal subtle effects such as how different forms of Dark Matter may modify structure growth. Currently most interesting is the damping effect of structure growth by massive neutrinos. Different observations of low redshift matter density fluctuations provided evidence for a signature of massive neutrinos. Here we discuss the study of the cosmic large-scale structure with a complete sample of nearby, X-ray luminous clusters from our REFLEX cluster survey. From the observed X-ray luminosity function and its reproduction for different cosmological models, we obtain tight constraints on the cosmological parameters describing the matter density, Omega_m, and the density fluctuation amplitude, sigma_8. A comparison of these constraints with the Planck results shows a discrepancy in the framework of a pure LambdaCDM model, but the results can be reconciled, if we allow for a neutrino mass in the range of 0.17 to 0.7 eV. Also some others, but not all of the observations of the nearby large-scale structure provide evidence or trends for signatures of massive neutrinos. With further improvement in the systematics and future survey projects, these indications will develop into a definitive measurement of neutrino masses., Comment: 10 pages, 4 figures

We study the effect of Active Nuclei Galaxy (AGN) feedback as one of the major mechanisms modifying the cluster morphology influencing scaling relations, which are the most uncertain factor in constraining cosmology with clusters of galaxies. Using cosmological hydrodynamical simulations we investigate how the AGN feedback changes the X-ray morphology of the simulated systems, and compare to the observed REXCESS (Representative XMM-Newton Cluster Structure Survey) clusters. We apply centre shifts and power ratios to characterise the cluster morphology, and find that our simulated clusters are more substructured than the observed ones. We show that the degree of this discrepancy is affected by the inclusion of AGN feedback. While the clusters simulated with the AGN feedback are in much better agreement with the REXCESS L_X-T relation, they are also more substructured, which increases the tension with observations. This suggests that not only global cluster properties such as L_X and T and radial profiles should be used to compare and to calibrate simulations with observations, but also substructure measures such as centre shifts and power ratios. We discuss what changes in the simulations might ease the tension with observational constraints on these quantities., Comment: 7 pages, 6 figures, accpeted for publication in A&A

We release the next installment of the Stripe 82 X-ray survey point-source catalog, which currently covers 31.3 deg$^2$ of the Sloan Digital Sky Survey (SDSS) Stripe 82 Legacy field. In total, 6181 unique X-ray sources are significantly detected with {\it XMM-Newton} ($>5\sigma$) and {\it Chandra} ($>4.5\sigma$). This catalog release includes data from {\it XMM-Newton} cycle AO 13, which approximately doubled the Stripe 82X survey area. The flux limits of the Stripe 82X survey are $8.7\times10^{-16}$ erg s$^{-1}$ cm$^{-2}$, $4.7\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$, and $2.1\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the soft (0.5-2 keV), hard (2-10 keV), and full bands (0.5-10 keV), respectively, with approximate half-area survey flux limits of $5.4\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$, $2.9\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$, and $1.7\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$. We matched the X-ray source lists to available multi-wavelength catalogs, including updated matches to the previous release of the Stripe 82X survey; 88\% of the sample is matched to a multi-wavelength counterpart. Due to the wide area of Stripe 82X and rich ancillary multi-wavelength data, including coadded SDSS photometry, mid-infrared {\it WISE} coverage, near-infrared coverage from UKIDSS and VHS, ultraviolet coverage from {\it GALEX}, radio coverage from FIRST, and far-infrared coverage from {\it Herschel}, as well as existing $\sim$30\% optical spectroscopic completeness, we are beginning to uncover rare objects, such as obscured high-luminosity AGN at high-redshift. The Stripe 82X point source catalog is a valuable dataset for constraining how this population grows and evolves, as well as for studying how they interact with the galaxies in which they live., Comment: accepted for publication in ApJ; 23 pages (emulateapj)

The presence of hot gaseous coronae around present-day massive spiral galaxies is a fundamental prediction of galaxy formation models. However, our observational knowledge remains scarce, since to date only four gaseous coronae were detected around spirals with massive stellar bodies ($\gtrsim2\times10^{11} \ \rm{M_{\odot}}$). To explore the hot coronae around lower mass spiral galaxies, we utilized Chandra X-ray observations of a sample of eight normal spiral galaxies with stellar masses of $(0.7-2.0)\times10^{11} \ \rm{M_{\odot}}$. Although statistically significant diffuse X-ray emission is not detected beyond the optical radii ($\sim20$ kpc) of the galaxies, we derive $3\sigma$ limits on the characteristics of the coronae. These limits, complemented with previous detections of NGC 1961 and NGC 6753, are used to probe the Illustris Simulation. The observed $3\sigma$ upper limits on the X-ray luminosities and gas masses exceed or are at the upper end of the model predictions. For NGC 1961 and NGC 6753 the observed gas temperatures, metal abundances, and electron density profiles broadly agree with those predicted by Illustris. These results hint that the physics modules of Illustris are broadly consistent with the observed properties of hot coronae around spiral galaxies. However, a shortcoming of Illustris is that massive black holes, mostly residing in giant ellipticals, give rise to powerful radio-mode AGN feedback, which results in under luminous coronae for ellipticals., Comment: 12 pages, 6 figures, accepted for publication in ApJ

To obtain a physically well-motivated definition of superclusters, we proposed in our previous work to select superclusters with an overdensity criterion that selects only those objects that will collapse in the future, including those that are at a turn-around in the present epoch. In this paper we present numerical values for these criteria for a range of standard cosmological models. We express these criteria in terms of a density ratio or, alternatively, as an infall velocity and show that these two criteria give almost identical results. To better illustrate the implications of this definition, we applied our criteria to some prominent structures in the local Universe, the Local supercluster, Shapley supercluster, and the recently reported Laniakea supercluster to understand their future evolution. We find that for the Local and Shapley superclusters, only the central regions will collapse in the future, while Laniakea does not constitute a significant overdensity and will disperse in the future. Finally, we suggest that those superclusters that will survive the accelerating cosmic expansion and collapse in the future be called "superstes-clusters", where "superstes" means survivor in Latin, to distinguish them from traditional superclusters., Comment: Accepted for publication as Letter in A&A, 6 pages

Clusters of galaxies are important probes for the large-scale structure that allow us to test cosmological models. With the REFLEX II galaxy cluster survey we previously derived tight constraints on the cosmological parameters for the matter density, Omega_m, and the amplitude parameter of the matter density fluctuations, sigma_8. Whereas in these previous studies no effect of massive neutrinos was taken into account, we explore these effects in the present publication. We derive cosmological constraints for the sum of the neutrino masses of the conventional three neutrino families in the range 0 to 0.6 eV. The influence on the constraints of Omega_m and sigma_8 for the expected mass range is weak. Interesting constraints on the neutrino properties can be derived by comparing the cluster data with those from the Planck cosmic microwave background observations. The current tension between the Planck results and clusters can formally be resolved with neutrino masses of about M_nu = 0.45 (+- 0.28, 1-sigma) eV. While we caution not to consider this a firm measurement because it might also be the result of unresolved systematics, it is interesting that other measurements of the local large-scale structure fluctuation amplitude, like that of cosmic lensing shear, yield similar results and additionally confirm the effect of massive neutrinos. Among the indicators for massive neutrinos, galaxy clusters and in particular our large and well-controlled cluster survey currently provide the best potential for constraints of the total neutrino mass., Comment: Accepted for publication as Letter in Astronomy and Astrophysics, 5 pages, 3 figures

Galaxy clusters' structure, dominated by dark matter, is traced by member galaxies in the optical and hot intra-cluster medium (ICM) in X-rays. We compare the radial distribution of these components and determine the mass-to-light ratio vs. system mass relation. We use 14 clusters from the REXCESS sample which is representative of clusters detected in X-ray surveys. Photometric observations with the Wide Field Imager on the 2.2m MPG/ESO telescope are used to determine the number density profiles of the galaxy distribution out to $r_{200}$. These are compared to electron density profiles of the ICM obtained using XMM-Newton, and dark matter profiles inferred from scaling relations and an NFW model. While red sequence galaxies trace the total matter profile, the blue galaxy distribution is much shallower. We see a deficit of faint galaxies in the central regions of massive and regular clusters, and strong suppression of bright and faint blue galaxies in the centres of cool-core clusters, attributable to ram pressure stripping of gas from blue galaxies in high density regions of ICM and disruption of faint galaxies due to galaxy interactions. We find a mass-to-light ratio vs. mass relation within $r_{200}$ of $\left(3.0\pm0.4\right) \times 10^2\, h\,\mathrm{M}_{\odot}\,\mathrm{L}_{\odot}^{-1}$ at $10^{15}\,\mathrm{M}_{\odot}$ with slope $0.16 \pm 0.14$, consistent with most previous results.

We report the discovery of the merging cluster, RXCJ2359.3-6042, from the REFLEX II cluster survey and present our results from all three detectors combined in the imaging and spectral analysis of the XMM-Newton data. Also known as Abell 4067, this is a unique system, where a compact bullet penetrates an extended, low density cluster at redshift z=0.099 clearly seen from our follow-up XMM-Newton observation. The bullet goes right through the central region of the cluster without being disrupted and we can clearly watch the process how the bullet component is stripped of its layers outside the core. There is an indication of a shock heated region in the East of the cluster with a higher temperature. The bulk temperature of the cluster is about 3.12 keV implying a lower mass system. Spearheading the bullet is a cool core centred by a massive early type galaxy. The temperatures and metallicities of a few regions in the cluster derived from the spectral analysis supports our conjecture based on the surface brightness image that a much colder compact component at 1.55 keV with large metallicity (0.75 Zsol) penetrates the main cluster, where the core of the infalling component survived the merger leaving stripped gas behind at the centre of the main cluster. We also give an estimate of the total mass within r500, which is about 2e14Msol from the deprojected spherical-beta modelling of the cluster in good agreement with other mass estimates from the M--Tx and M-sigma_v relations., Comment: 5 pages, 4 figures, A&A in press. Images with better resolution will be available through the journal

Dusty, star-forming galaxies have a critical role in the formation and evolution of massive galaxies in the Universe. Using deep far-infrared imaging in the range 100-500um obtained with the Herschel telescope, we investigate the dust-obscured star formation in the galaxy cluster XDCP J0044.0-2033 at z=1.58, the most massive cluster at z >1.5, with a measured mass M200= 4.7x10$^{14}$ Msun. We perform an analysis of the spectral energy distributions (SEDs) of 12 cluster members (5 spectroscopically confirmed) detected with >3$\sigma$ significance in the PACS maps, all ULIRGs. The individual star formation rates (SFRs) lie in the range 155-824 Ms/yr, with dust temperatures of 24$\pm$35 K. We measure a strikingly high amount of star formation (SF) in the cluster core, SFR (< 250 kpc) > 1875$\pm$158 Ms/yr, 4x higher than the amount of star formation in the cluster outskirts. This scenario is unprecedented in a galaxy cluster, showing for the first time a reversal of the SF-density relation at z~1.6 in a massive cluster., Comment: Letter accepted for publication in MNRAS, ESA Press Release on 18 December 2014

Several claims have been made that we are located in a locally underdense region of the Universe based on observations of supernovae and galaxy density distributions. Two recent studies of K-band galaxy surveys have provided new support for a local underdensity in the galaxy distribution out to distances of 200 - 300 Mpc. If confirmed, such large local underdensities would have important implications on the interpretation of local measurements of cosmological parameters. Galaxy clusters have been shown to be ideal probes to trace the large-scale structure of the Universe. In this paper we study the local density distribution in the southern sky with the X-ray detected galaxy clusters from the REFLEX II cluster survey. From the normalized comoving number density of clusters we find an average underdensity of ~30 - 40% in the redshift range out to z ~0.04 (~170 Mpc) in the southern extragalactic sky with a significance larger than 3.4sigma. On larger scales from 300 Mpc to over 1 Gpc the density distribution appears remarkably homogeneous. The local underdensity seems to be dominated by the South Galactic Cap region. A comparison of the cluster distribution with that of galaxies in the K-band from a recent study shows that galaxies and clusters trace each other very closely in density. In the South Galactic Cap region both surveys find a local underdensity in the redshift range z= 0 to 0.05 and no significant underdensity in the North Galactic Cap at southern latitudes. Our results to not support cosmological models that attempt to interpret the cosmic acceleration by a large local void, since the local underdensity we find is not isotropic and limited to a size significantly smaller than 300 Mpc radius., Comment: submitted to Astronomy and Astrophysics, 8 pages, 9 figures

Superclusters are the largest, observed matter density structures in the Universe. Recently Chon et al.(2013) presented the first supercluster catalogue constructed with a well-defined selection function based on the X-ray flux-limited cluster survey, REFLEX II. For the construction of the sample we proposed a concept to find the large objects with a minimum overdensity such that most of their mass will collapse in the future. The main goal of the paper is to provide support for our concept using simulations that we can, on the basis of our observational sample of X-ray clusters, construct a supercluster sample defined by a certain minimum overdensity, and to test how superclusters trace the underlying dark matter distribution. Our results confirm that an overdensity in the number of clusters is tightly correlated with an overdensity of the dark matter distribution. This enables us to define superclusters such that most of the mass will collapse in the future and to get first-order mass estimates of superclusters on the basis of the properties of the member clusters. We also show that in this context the ratio of the cluster number density and dark matter mass density is consistent with the theoretically expected cluster bias. Our previous work provided evidence that superclusters are a special environment for density structures of the dark matter to grow differently from the field as characterised by the X-ray luminosity function. Here we confirm for the first time that this originates from a top-heavy mass function at high statistical significance provided by a Kolmogorov-Smirnov test. We also find in close agreement with observations that the superclusters occupy only a small volume of few percent while they contain more than half of the clusters in the present day Universe., Comment: 11 pages, 11 figures, accepted for publication in Astronomy & Astrophysics

In a previous paper we investigated the energy transfer of massive stars to the interstellar medium as a function of time and the geometrical configuration of three massive stars via 3D-mesh-refining hydrodynamics simulations, following the complete evolution of the massive stars and their supernovae except non-thermal processes . We analysed our ISM simulation results with the help of spectra for plasma temperatures between 0.1 and 10 keV and computed the spectral evolution and the spatio-temporal distribution of the hot gas. Results. Despite significant input of high temperature gas from supernovae and fast stellar winds, the resulting thermal X-ray spectra are generally very soft, with most of the emission well below 1 keV. We show that this is due to mixing triggered by resolved hydrodynamic instabilities. Supernovae enhance the X-ray luminosity of a superbubble by 1-2 orders of magnitude for a time span of about 0.1 Myr; longer if a supernova occurs in a larger superbubble and shorter in higher energy bands. Peak superbubble luminosities of the order of 10^{36} erg/s are reproduced well. The strong decay of the X-ray luminosity is due to bubble expansion, hydrodynamic instabilities related to the acceleration of the superbubble's shell thanks to the sudden energy input, and subsequent mixing. We also find global oscillations of our simulated superbubbles, which produce spatial variations of the X-ray spectrum, similar to what we see in the Orion-Eridanus cavity. We calculated the fraction of energy emitted in X-rays and find that with a value of a few times 10^{-4}, it is about a factor of ten below the measurements for nearby galaxies., Comment: 13 pages 12 figures, accepted by Astronomy & Astrophysics

Galaxy clusters provide unique laboratories to study astrophysical processes on large scales and are important probes for cosmology. X-ray observations are currently the best means of detecting and characterizing galaxy clusters. In this paper we describe the construction of the REFLEX II galaxy cluster survey based on the southern part of the ROSAT All-Sky Survey. REFLEX II extends the REFLEX I survey by a factor of about two down to a flux limit of $1.8 \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ (0.1 - 2.4 keV). We describe the determination of the X-ray parameters, the process of X-ray source identification, and the construction of the survey selection function. The REFLEX II cluster sample comprises currently 915 objects. A standard selection function is derived for a lower source count limit of 20 photons in addition to the flux limit. The median redshift of the sample is $z = 0.102$. Internal consistency checks and the comparison to several other galaxy cluster surveys imply that REFLEX II is better than 90\% complete with a contamination less than 10\%., Comment: Astronomy and Astrophysics Vol. 555, A30 - 15 pages, 20 figures

Clusters of galaxies as the largest clearly defined objects in our Universe are ideal laboratories to study the distribution of the most abundant chemical elements heavier than hydrogen and helium and the history of their production. The cluster environment allows us to study the element abundances not only inside the galaxies, but also in the intergalactic space, the intracluster medium. Since the intracluster medium is heated to temperatures of several ten Million degrees, we can study the chemical composition of this medium through X-ray spectroscopy. Up to 13 heavy elements have been detected by X-ray spectroscopy so far. The element most easily detected in the X-ray spectra is iron. In massive galaxy clusters we find a larger mass of heavy elements in the intracluster medium than in the galaxies. The consideration of the intracluster medium is therefore vital for an understanding of the complete history of nucleosynthesis of the heavy elements. The observed abundances for all elements heavier than nitrogen can roughly be modeled by using two types of sources: core collapse supernovae and supernovae type Ia. So called cool-core galaxy clusters show a larger heavy element abundance in the cluster center which seems to be enriched primarily by products of supernovae of type Ia. The evidence for observations of an evolution of the heavy element abundance with redshift has still a moderate significance., Comment: To be published by Mem. S.A.It., Conference Proceedings of the Rencontre de l'Observatoire 2013 ESO Workshop on Metal Production in a Hierachical Universe, 8 pages, 9 figures

We report a systematic multi-wavelength investigation of environments of the brightest cluster galaxies (BCGs), using the X-ray data from the Chandra archive, and optical images taken with 34'x 27' field-of-view Subaru Suprime-Cam. Our goal is to help understand the relationship between the BCGs and their host clusters, and between the BCGs and other galaxies, to eventually address a question of the formation and co-evolution of BCGs and the clusters. Our results include: 1) Morphological variety of BCGs, or the second or the third brightest galaxy (BCG2, BCG3), is comparable to that of other bright red sequence galaxies, suggesting that we have a continuous variation of morphology between BCGs, BCG2, and BCG3, rather than a sharp separation between the BCG and the rest of the bright galaxies. 2) The offset of the BCG position relative to the cluster centre is correlated to the degree of concentration of cluster X-ray morphology (Spearman rho = -0.79), consistent with an interpretation that BCGs tend to be off-centered inside dynamically unsettled clusters. 3) Morphologically disturbed clusters tend to harbour the brighter BCGs, implying that the "early collapse" may not be the only major mechanism to control the BCG formation and evolution., Comment: Accepted for publication in NNRAS. 15 figures

Observations and cosmological simulations show galaxy clusters as a family of nearly self-similar objects with properties that can be described by scaling relations as a function of e.g. mass and time. Here we study the scaling relations between the galaxy velocity dispersion and X-ray quantities like X-ray bolometric luminosity and temperature in galaxy clusters at high redshifts (0.64 $\leq$ z $\leq$ 1.46). We also compare our results with the similar study of the local HIFLUGCS sample. For the analysis, we use a set of 15 distant galaxy clusters extracted from the literature plus a sample of 10 newly discovered clusters selected in X-rays by the \XMM Distant Cluster Project (XDCP) with more than 10 confirmed spectroscopic members per cluster. We also study the evolution of this scaling relation by comparing the high redshift results with the data from the local HIFLUGCS sample. We also investigated the $L_X - T_X$ and the $\sigma_v - T_X$ relations for the 15 clusters in the literature sample. We report the results of the X-ray and kinematic analysis of 10 newly detected high redshift clusters and provide their spectroscopic and kinematic details. For the entire, distant sample we find a slope fully consistent with the one typical of local clusters, albeit with a large associated uncertainty. The study on the evolution of the amplitude reveals a positive offset if the self-similar evolution is neglected, hence possibly indicating the need for including evolutionary effects. However, the $L_X - T_X$ relation is found to be in good agreement with the local relation without any significant redshift evolution. Finally, the $\sigma_v - T_X$ relation appears to slightly deviate from the theoretical expectation that galaxies and gas particles have a similar specific kinetic energy. However, the associated uncertainty is currently too large for making any conclusive statement in this regard., Comment: 24 pages, 20 figures, 11 tables, accepted for publication in A&A (v2: typos/language style corrections, updated references)

We have analyzed the {\it XMM-Newton} and {\it Chandra} data overlapping $\sim$16.5 deg$^2$ of Sloan Digital Sky Survey Stripe 82, including $\sim$4.6 deg$^2$ of proprietary {\it XMM-Newton} data that we present here. In total, 3362 unique X-ray sources are detected at high significance. We derive the {\it XMM-Newton} number counts and compare them with our previously reported {\it Chandra} Log$N$-Log$S$ relations and other X-ray surveys. The Stripe 82 X-ray source lists have been matched to multi-wavelength catalogs using a maximum likelihood estimator algorithm. We discovered the highest redshift ($z=5.86$) quasar yet identified in an X-ray survey. We find 2.5 times more high luminosity (L$_x \geq 10^{45}$ erg s$^{-1}$) AGN than the smaller area {\it Chandra} and {\it XMM-Newton} survey of COSMOS and 1.3 times as many identified by XBo\"otes. Comparing the high luminosity AGN we have identified with those predicted by population synthesis models, our results suggest that this AGN population is a more important component of cosmic black hole growth than previously appreciated. Approximately a third of the X-ray sources not detected in the optical are identified in the infrared, making them candidates for the elusive population of obscured high luminosity AGN in the early universe., Comment: 25 pages, 18 figures, 5 tables. accepted for publication in MNRAS. Catalogs can be downloaded at http://www.astro.yale.edu/lamassa/s82x.html

The goal of this paper is twofold. Firstly, we present F-VIPGI, a new version of the VIMOS Interactive Pipeline and Graphical Interface (VIPGI) adapted to handle FORS2 spectroscopic data. Secondly, we investigate the spectro-photometric properties of a sample of galaxies residing in distant X-ray selected galaxy clusters, the optical spectra of which were reduced with this new pipeline. We provide basic technical information about the innovations of the new software and, as a demonstration of the capabilities of the new pipeline, we show results obtained for 16 distant (0.65 < z < 1.25) X-ray luminous galaxy clusters selected within the XMM-Newton Distant Cluster Project. We performed a spectral indices analysis of the extracted optical spectra of their members, based on which we created a library of composite high signal-to-noise ratio spectra representative of passive and star-forming galaxies residing in distant galaxy clusters. The spectroscopic templates are provided to the community in electronic form. By comparing the spectro-photometric properties of our templates with the local and distant galaxy population residing in different environments, we find that passive galaxies in clusters appear to be well evolved already at z = 0.8 and even more so than the field galaxies at similar redshift. Even though these findings would point toward a significant acceleration of galaxy evolution in densest environments, we cannot exclude the importance of the mass as the main evolutionary driving element either. The latter effect may indeed be justified by the similarity of our composite passive spectrum with the luminous red galaxies template at intermediate redshift., Comment: 15 pages, 15 figures, in press on Astronomy and Astrophysics

We present the first supercluster catalogue constructed with the extended ROSAT-ESO Flux Limited X-ray Galaxy Cluster survey (REFLEX II) data, which comprises 919 X-ray selected galaxy clusters. Based on this cluster catalogue we construct a supercluster catalogue using a friends-of-friends algorithm with a linking length depending on the local cluster density. The resulting catalogue comprises 164 superclusters at redshift z<=0.4. We study the properties of different catalogues such as the distributions of the redshift, extent and multiplicity by varying the choice of parameters. In addition to the main catalogue we compile a large volume-limited cluster sample to investigate the statistics of the superclusters. We also compare the X-ray luminosity function for the clusters in superclusters with that for the field clusters with the flux- and volume-limited catalogues. The results mildly support the theoretical suggestion of a top-heavy X-ray luminosity function of galaxy clusters in regions of high cluster density., Comment: 16 pages, 11 figures, accepted by MNRAS

Galaxy clusters are one of the important cosmological probes to test the consistency of the observable structure and evolution of our Universe with the predictions of specific cosmological models. We use results from our analysis of the X-ray flux-limited REFLEX cluster sample from the ROSAT All-Sky Survey to illustrate the constraints on cosmological parameters that can be achieved with this approach. The upcoming eROSITA project of the Spektrum-Roentgen-Gamma mission will increase these capabilities by two orders of magnitude and importantly also increase the redshift range of such studies. We use the projected instrument performance to make predictions on the scope of the eROSITA survey and the potential of its exploitation., Comment: 5 pages, 8 figures, accepted for publication in Astronomische Nachrichten; the proceedings of the XMM-Newton Science Workshop: "Galaxy Clusters as Giant Cosmic Laboratories" at ESAC, Madrid, Spain, 21-23 May 2012

We present a morphological study of 35 X-ray luminous galaxy clusters at 0.15

Non-thermal motions in the intra-cluster medium (ICM) are believed to play a non-negligible role in the pressure support to the total gravitating mass of galaxy clusters. Future X-ray missions, such as ASTRO-H and ATHENA, will eventually allow us to directly detect the signature of these motions from high-resolution spectra of the ICM. In this paper, we present a study on a set of clusters extracted from a cosmological hydrodynamical simulation, devoted to explore the role of non-thermal velocity amplitude in characterising the cluster state and the relation between observed X-ray properties. In order to reach this goal, we apply the X-ray virtual telescope PHOX to generate synthetic observations of the simulated clusters with both Chandra and ATHENA, the latter used as an example for the performance of very high-resolution X-ray telescopes. From Chandra spectra we extract global properties, e.g. luminosity and temperature, and from ATHENA spectra we estimate the gas velocity dispersion along the line of sight from the broadening of heavy-ion emission lines (e.g. Fe). We further extend the analysis to the relation between non-thermal velocity dispersion of the gas and the L_X-T scaling law for the simulated clusters. Interestingly, we find a clear dependence of slope and scatter on the selection criterion for the clusters, based on the level of significance of non-thermal motions. Namely, the scatter in the relation is significantly reduced by the exclusion of the clusters, for which we estimate the highest turbulent velocities. Such velocity diagnostics appears therefore as a promising independent way to identify disturbed clusters, in addition to the commonly used morphological inspection., Comment: 17 pages, 15 figures, abstract abridged. Accepted for publication in MNRAS

We present the first results of a wide area X-ray survey within the Sloan Digital Sky Survey (SDSS) Stripe 82, a 300 deg$^2$ region of the sky with a substantial investment in multi-wavelength coverage. We analyzed archival {\it Chandra} observations that cover 7.5 deg$^2$ within Stripe 82 ("Stripe 82 ACX"), reaching 4.5$\sigma$ flux limits of 7.9$\times10^{-16}$, 3.4$\times10^{-15}$ and 1.8$\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ in the soft (0.5-2 keV), hard (2-7 keV) and full (0.5-7 keV) bands, to find 774, 239 and 1118 X-ray sources, respectively. Three hundred twenty-one sources are detected only in the full band and 9 sources are detected solely in the soft band. Utilizing data products from the {\it Chandra} Source Catalog, we construct independent Log$N$-Log$S$ relationships, detailing the number density of X-ray sources as a function of flux, which show general agreement with previous {\it Chandra} surveys. We compare the luminosity distribution of Stripe 82 ACX with the smaller, deeper CDF-S + E-CDFS surveys and with {\it Chandra}-COSMOS, illustrating the benefit of wide-area surveys in locating high luminosity AGN. We also investigate the differences and similarities of X-ray and optical selection to uncover obscured AGN in the local Universe. Finally, we estimate the population of AGN we expect to find with increased coverage of 100 deg$^2$ or 300 deg$^2$, which will provide unprecedented insight into the high redshift, high luminosity regime of black hole growth currently under-represented in X-ray surveys., Comment: Accepted for publication in MNRAS, 15 pages, 6 Figures, 2 Tables

Cygnus A harbours the nearest powerful radio jet of an Fanaroff-Riley (FR) class II radio galaxy in a galaxy cluster where the interaction of the jet with the intracluster medium (ICM) can be studied in detail. We use a large set of Chandra archival data, VLA and new LOFAR observations to shed new light on the interaction of the jets with the ICM. We identify an X-ray cavity in the distribution of the X-ray emitting plasma in the region south of the Cyg A nucleus which has lower pressure than the surrounding medium. The LOFAR and VLA radio observations show that the cavity is filled with synchrotron emitting plasma. The spectral age and the buoyancy time of the cavity indicates an age at least as large as the current Cyg A jets and not much larger than twice this time. We suggest that this cavity was created in a previous active phase of Cyg A when the energy output of the Active Galactic Nucleus (AGN) was about two orders of magnitude less than today., Comment: Letter submitted on 4 May 2012 to A&A, 4 pages, 4 figures

The 4MOST consortium is currently halfway through a Conceptual Design study for ESO with the aim to develop a wide-field (>3 square degree, goal >5 square degree), high-multiplex (>1500 fibres, goal 3000 fibres) spectroscopic survey facility for an ESO 4m-class telescope (VISTA). 4MOST will run permanently on the telescope to perform a 5 year public survey yielding more than 20 million spectra at resolution R~5000 ({\lambda}=390-1000 nm) and more than 2 million spectra at R~20,000 (395-456.5 nm & 587-673 nm). The 4MOST design is especially intended to complement three key all-sky, space-based observatories of prime European interest: Gaia, eROSITA and Euclid. Initial design and performance estimates for the wide-field corrector concepts are presented. We consider two fibre positioner concepts, a well-known Phi-Theta system and a new R-Theta concept with a large patrol area. The spectrographs are fixed configuration two-arm spectrographs, with dedicated spectrographs for the high- and low-resolution. A full facility simulator is being developed to guide trade-off decisions regarding the optimal field-of-view, number of fibres needed, and the relative fraction of high-to-low resolution fibres. Mock catalogues with template spectra from seven Design Reference Surveys are simulated to verify the science requirements of 4MOST. The 4MOST consortium aims to deliver the full 4MOST facility by the end of 2018 and start delivering high-level data products for both consortium and ESO community targets a year later with yearly increments., Comment: To appear in the proceedings of the SPIE Astronomical Instrumentation + Telescopes conference, Amsterdam, 2012. 15 pages, 16 figures

We report 19 intermediate redshift clusters newly detected in the ROSAT All-Sky survey that are spectroscopically confirmed. They form a part of 911 objects in the REFLEX II cluster catalogue with a limiting flux of 1.8\times10^12 erg/s/cm2 in the 0.1-2.4 keV ROSAT band at redshift z >= 0.2. In addition we report three clusters from the REFLEX III supplementary catalogue, which contains objects below the REFLEX II flux limit but satisfies the redshift constraint above. These clusters are spectroscopically followed-up by our ESO NTT-EFOSC2 campaigns for the redshift measurement. We describe our observing and data reduction methods. We show how X-ray properties such as spectral hardness ratio and source extent can be used as important diagnostics in selecting galaxy cluster candidates. Physical properties of the clusters are subsequently calculated from the X-ray observations. This sample contains the high mass and intermediate-redshift galaxy clusters for astrophysical and cosmological applications., Comment: Astronomy and Astrophysics (in press)

The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass. A large effort is therefore undertaken to compile global galaxy cluster properties in particular obtained through X-ray observations and to study their scaling relations. However, the scaling schemes used in the literature differ. The present paper aims to clarify this situation by providing a thorough review of the scaling laws within the standard model of large-scale structure growth and to discus various steps of practical approximations. We derive the scaling laws for X-ray observables and cluster mass within the pure gravitational structure growth scenario. Using N-body simulations we test the recent formation approximation used in earlier analytic approaches which involves a redshift dependent overdensity parameter.We find this approximation less precise than the use of a fiducial radius based on a fixed overdensity with respect to critical density. Inspired by the comparison of the predicted scaling relations with observations we propose a first order modification of the scaling scheme to include the observed effects of hydrodynamics in structure formation. This modification involves a cluster mass dependent gas mass fraction. We also discuss the observational results of the reshift evolution of the most important scaling relations and find that also a redshift dependence of the gas mass to total mass relation has to be invoked within our modification scheme. We find that the current observational data are within their uncertainties consistent with the proposed modified scaling laws., Comment: Astronomy and Astrophysics (in press) 14 pages, 6 figures

We present a novel, virtual X-ray observatory designed to obtain synthetic observations from hydro-numerical simulations, named PHOX. In particular, we provide a description of the code constituting the photon simulator and of the new approach implemented. We apply PHOX to simulated galaxy clusters in order to demonstrate its capabilities. In fact, X-ray observations of clusters of galaxies continue to provide us with an increasingly detailed picture of their structure and of the underlying physical phenomena governing the gaseous component, which dominates their baryonic content. Therefore, it is fundamental to find the most direct and faithful way to compare such observational data with hydrodynamical simulations of cluster-like objects, which can currently include various complex physical processes. Here, we present and analyse synthetic Suzaku observations of two cluster-size haloes obtained by processing with PHOX the hydrodynamical simulation of the large-scale, filament-like region in which they reside. Taking advantage of the simulated data, we test the results inferred from the X-ray analysis of the mock observations against the underlying, known solution. Remarkably, we are able to recover the theoretical temperature distribution of the two haloes by means of the multi-temperature fitting of the synthetic spectra. Moreover, the shapes of the reconstructed distributions allow us to trace the different thermal structure that distinguishes the dynamical state of the two haloes., Comment: 13 pages, 4 figures, 2 tables. Accepted for publication in MNRAS

Brightest cluster galaxies (BCGs) in the cores of galaxy clusters have distinctly different properties from other low redshift massive ellipticals. The majority of the BCGs in cool-core clusters show signs of active star formation. We present observations of NGC 4696, the BCG of the Centaurus galaxy cluster, at far-infrared (FIR) wavelengths with the Herschel space telescope. Using the PACS spectrometer, we detect the two strongest coolants of the interstellar medium, CII at 157.74 micron and OI at 63.18 micron, and in addition NII at 121.90 micron. The CII emission is extended over a region of 7 kpc with a similar spatial morphology and kinematics to the optical H-alpha emission. This has the profound implication that the optical hydrogen recombination line, H-alpha, the optical forbidden lines, NII 6583 Angstrom, the soft X-ray filaments and the far-infrared CII line all have the same energy source. We also detect dust emission using the PACS and SPIRE photometers at all six wavebands. We perform a detailed spectral energy distribution fitting using a two-component modified black-body function and find a cold 19 K dust component with mass 1.6x10^6 solar mass and a warm 46 K dust component with mass 4.0x10^3 solar mass. The total FIR luminosity between 8 micron and 1000 micron is 7.5x10^8 solar luminosity, which using Kennicutt relation yields a low star formation rate of 0.13 solar mass per yr. This value is consistent with values derived from other tracers, such as ultraviolet emission. Combining the spectroscopic and photometric results together with optical H-alpha, we model emitting clouds consisting of photodissociation regions (PDRs) adjacent to ionized regions. We show that in addition to old and young stellar populations, there is another source of energy, such as cosmic rays, shocks or reconnection diffusion, required to excite the H-alpha and CII filaments., Comment: 17 pages, 9 figures, 5 tables, accepted for publication in MNRAS

We report on 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 for strong star formation activity in any of the objects, the color spread could indicate 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 ~ 20kpc. From the X-ray luminosity we estimate a cluster mass of M200 ~ 1.2 x 10^(14) Msun. The data seem to suggest a scenario in which XMMU J0338.8+0021 is a young system, possibly caught in a moment of active ongoing mass assembly., Comment: 6 pages, 4 figures, 2 Tables

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 (5sigma) as a compact, but significantly extended source in the X-ray data, with a soft-band flux f(r<40")=(1.5+-0.3)x10^(-14) erg/s/cm2. Optical/NIR follow-up observations confirmed the presence of an overdensity of red galaxies matching the X-ray emission. The cluster was spectroscopically confirmed to be at z=1.579 using ground-based VLT/FORS2 spectroscopy. The analysis of the I-H colour-magnitude diagram shows a sequence of red galaxies with a colour range [3.7 < I-H < 4.6] within 1' from the cluster X-ray emission peak. However, the three spectroscopic members (all with complex morphology) have significantly bluer colours relative to the observed red-sequence. In addition, two of the three cluster members have [OII] emission, indicative of on-going star formation. Using the spectroscopic redshift we estimated the X-ray bolometric luminosity, Lbol = 5.8x10^44 erg/s, implying a massive galaxy cluster. This places XMMU J0044.0-2033 at the forefront of massive distant clusters, closing the gap between lower redshift systems and recently discovered proto- and low-mass clusters at z >1.6., Comment: letter to appear in A&A