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Distant galaxy clusters
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Title: Distant galaxy clusters in the XMM Large Scale Structure survey
Authors: J.P. Willis, N. Clerc, M.N. Bremer, M. Pierre, C. Adami, O. Ilbert, B. Maughan, S. Maurogordato, F. Pacaud, I. Valtchanov, L. Chiappetti, K. Thanjavur, S. Gwyn, E.R. Stanway, C. Winkworth

Distant galaxy clusters provide important tests of the growth of large scale structure in addition to highlighting the process of galaxy evolution in a consistently defined environment at large look back time. We present a sample of 22 distant (z>0.8) galaxy clusters and cluster candidates selected from the 9 deg2 footprint of the overlapping X-ray Multi Mirror (XMM) Large Scale Structure (LSS), CFHTLS Wide and Spitzer SWIRE surveys. Clusters are selected as extended X-ray sources with an accompanying overdensity of galaxies displaying optical to mid-infrared photometry consistent with z>0.8. Nine clusters have confirmed spectroscopic redshifts in the interval 0.8<z<1.2, four of which are presented here for the first time. A further 11 candidate clusters have between 8 and 10 band photometric redshifts in the interval 0.8<z<2.2, while the remaining two candidates do not have information in sufficient wavebands to generate a reliable photometric redshift. All of the candidate clusters reported in this paper are presented for the first time. Those confirmed and candidate clusters with available near infrared photometry display evidence for a red sequence galaxy population, determined either individually or via a stacking analysis, whose colour is consistent with the expectation of an old, coeval stellar population observed at the cluster redshift. We further note that the sample displays a large range of red fraction values indicating that the clusters may be at different stages of red sequence assembly. We compare the observed X-ray emission to the flux expected from a suite of model clusters and find that the sample displays an effective mass limit M200 ~ 1e14 Msolar with all clusters displaying masses consistent with M200 < 5e14 Msolar. This XMM distant cluster study represents a complete sample of X-ray selected z>0.8 clusters.

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RE: Galaxy clusters
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Title: The Dark Matter Haloes of Chandra X-ray Galaxy Clusters and Baryons Effect
Authors: Iu. Babyk, I. Vavilova, A. Del Popolo

We present results based on Chandra observations of a large sample of 129 hot galaxy clusters. We measure the concentration parameter c_200, the dark mass M_200 and the baryonic mass content in all the objects of our sample, providing the largest dataset of mass parameters for galaxy clusters in the redshift range z = 0.01 -- 1.4. We confirm that a tight correlation between c_200 and M_200, c \propto M^a_{vir}/(1+z)^b with a = -0.56 ± 0.15 and b = 0.80 ± 0.25 (68 per cent confidence limits), is present, in good agreement with the predictions from numerical simulations and previous observations.
The inner slope \alpha of the total mass density profile (rho(r)\propto r^{-\alpha}) is derived from the slope of the integrated mass profile, using a generalised NFW model. The values of the inner slope \alpha, is \alpha = 0.94 ± 0.13. Finally, we show that the inner slope of the density profile, \alpha correlates with the baryonic mass content, M_b: namely \alpha is decreasing with increasing baryonic mass content.

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Title: Formation of Galaxy Clusters
Authors: Andrey Kravtsov, Stefano Borgani

In this review, we describe our current understanding of cluster formation: from the general picture of collapse from initial density fluctuations in an expanding Universe to detailed simulations of cluster formation including the effects of galaxy formation. We outline both the areas in which highly accurate predictions of theoretical models can be obtained and areas where predictions are uncertain due to uncertain physics of galaxy formation and feedback. The former includes the description of the structural properties of the dark matter halos hosting cluster, their mass function and clustering properties. Their study provides a foundation for cosmological applications of clusters and for testing the fundamental assumptions of the standard model of structure formation. The latter includes the description of the total gas and stellar fractions, the thermodynamical and non-thermal processes in the intracluster plasma. Their study serves as a testing ground for galaxy formation models and plasma physics. In this context, we identify a suitable radial range where the observed thermal properties of the intra-cluster plasma exhibit the most regular behaviour and thus can be used to define robust observational proxies for the total cluster mass. We put particular emphasis on examining assumptions and limitations of the widely used self-similar model of clusters. Finally, we discuss the formation of clusters in non-standard cosmological models, such as non-Gaussian models for the initial density field and models with modified gravity, along with prospects for testing these alternative scenarios with large cluster surveys in the near future.

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Title: A journey from the outskirts to the cores of groups I: Colour- and mass-segregation in 20K-zCOSMOS groups
Authors: V. Presotto, A. Iovino, M. Scodeggio, O. Cucciati, C. Knobel, M. Bolzonella, P. Oesch, A. Finoguenov, M. Tanaka, K. Kovac, Y. Peng, G. Zamorani, S. Bardelli, L. Pozzetti, P. Kampczyk, C. López-Sanjuan, D. Vergani, E. Zucca, L. A. M. Tasca, C. M. Carollo, T. Contini, J.-P. Kneib, O. Le Fèvre, S. Lilly, V. Mainieri, A. Renzini, A. Bongiorno, K. Caputi, S. de la Torre, L. de Ravel, P. Franzetti, B. Garilli, F. Lamareille, J.-F. Le Borgne, V. Le Brun, C. Maier, M. Mignoli, R. Pellò, E. Perez-Montero, E. Ricciardelli, J. D. Silverman, L. Tresse, L. Barnes, R. Bordoloi, A. Cappi, A. Cimatti, G. Coppa, A. M. Koekemoer, H. J. McCracken, M. Moresco, P. Nair, N. Welikala

Using the group catalogue obtained from zCOSMOS spectroscopic data and the complementary photometric data from the COSMOS survey, we explore segregation effects occurring in groups of galaxies at intermediate/high redshifts. We built two composite groups at intermediate (0.2 <= z <= 0.45) and high (0.45 < z <= 0.8) redshifts, and we divided the corresponding composite group galaxies into three samples according to their distance from the group center. We explored how galaxy stellar masses and colours - working in narrow bins of stellar masses - vary as a function of the galaxy distance from the group center. We found that the most massive galaxies in our sample (Log(Galaxy mass/solar mass) >= 10.6) do not display any strong group-centric dependence of the fractions of red/blue objects. For galaxies of lower masses (9.8 <= Log(Galaxy mass/solar mass) <= 10.6) there is a radial dependence in the changing mix of red and blue galaxies. This dependence is most evident in poor groups, whereas richer groups do not display any obvious trend of the blue fraction. Interestingly, mass segregation shows the opposite behaviour: it is visible only in rich groups, while poorer groups have a a constant mix of galaxy stellar masses as a function of radius. We suggest a simple scenario where colour- and mass-segregation originate from different physical processes. While dynamical friction is the obvious cause for establishing mass segregation, both starvation and galaxy-galaxy collisions are plausible mechanisms to quench star formation in groups at a faster rate than in the field. In poorer groups the environmental effects are caught in action superimposed to secular galaxy evolution. Their member galaxies display increasing blue fractions when moving from the group center to more external regions, presumably reflecting the recent accretion history of these groups.

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Title: An XMM-Newton spatially-resolved study of metal abundance evolution in distant galaxy clusters
Authors: Alessandro Baldi, Stefano Ettori, Silvano Molendi, Italo Balestra, Fabio Gastaldello, Paolo Tozzi

We present an XMM-Newton analysis of the X-ray spectra of 39 clusters of galaxies at 0.4<z<1.4, covering a temperature range of 1.5<=kT<=11 keV. We performed a spatially resolved spectral analysis to study how the abundance evolves with redshift not only through a single emission measure performed on the whole cluster but also spatially resolving the cluster emission. We do not observe a statistically significant (>2sigma) abundance evolution with redshift. The most significant deviation from no evolution (90% c.l.) is observed in the emission from the whole cluster (r<0.6r500), that could be parameterised as Z=A*(1+z)^(-0.8+/-0.5). Dividing the emission in 3 radial bins, no significant evidence of abundance evolution could be observed fitting the data with a power-law. A substantial agreement with measures presented in previous works is found. The error-weighted mean of the spatially resolved abundances in 3 redshift bins is consistent to be constant with z. Although the large error bars in the measure of the weighted-mean abundance prevent us from claiming any significant spatially resolved evolution, the trend with z in the 0.15-0.4r500 radial bin complements nicely the measures of Maughan et al., and broadly agrees with theoretical predictions. We also found that the data points derived from the spatially resolved analysis are well fitted by the relation Z(r,z)=Z0*(1+(r/0.15r500)^2)^(-a)*((1+z)/1.6)^(-gamma), showing a significant negative trend of Z with the radius and no significant evolution with the redshift. The present study is the first attempt made to spatially resolve the evolution of abundance with redshift. However, the sample size and the low statistics associated with most of the clusters in the sample prevents us to draw any statistically significant conclusion on the different evolutionary path that the different regions of the clusters may have traversed.

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Professor Gillian Wilson on Astronomy

Astronomer Gillian Wilson of the University of California, Riverside talks about galaxy clusters, including very distant ones. She is the leader of an international team of researchers that completed the largest ever survey designed to find very distant clusters of galaxies.



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Title: A massive proto-cluster of galaxies at a redshift of z {\approx} 5.3
Authors: Peter L. Capak, Dominik Riechers, Nick Z. Scoville, Chris Carilli, Pierre Cox, Roberto Neri, Brant Robertson, Mara Salvato, Eva Schinnerer, Lin Yan, Grant W. Wilson, Min Yun, Francesca Civano, Martin Elvis, Alexander Karim, Bahram Mobasher, Johannes G. Staguhn

Massive clusters of galaxies have been found as early as 3.9 Billion years (z=1.62) after the Big Bang containing stars that formed at even earlier epochs. Cosmological simulations using the current cold dark matter paradigm predict these systems should descend from "proto-clusters" - early over-densities of massive galaxies that merge hierarchically to form a cluster. These proto-cluster regions themselves are built-up hierarchically and so are expected to contain extremely massive galaxies which can be observed as luminous quasars and starbursts. However, observational evidence for this scenario is sparse due to the fact that high-redshift proto-clusters are rare and difficult to observe. Here we report a proto-cluster region 1 billion years (z=5.3) after the Big Bang. This cluster of massive galaxies extends over >13 Mega-parsecs, contains a luminous quasar as well as a system rich in molecular gas. These massive galaxies place a lower limit of >4x10^11 solar masses of dark and luminous matter in this region consistent with that expected from cosmological simulations for the earliest galaxy clusters.

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Bubble-blowing black holes explain stellar dearth

Black holes blowing huge bubbles may explain the lack of star formation in the cores of galaxy clusters.
Gas at the centre of galaxy clusters should be cooling as it loses energy; this would allow nearby material to compress the gas and create ideal conditions for making stars. But giant "bubbles" of hot, low-density gas may be dragging away the cool gas, say Edward Pope of the University of Victoria in British Columbia, Canada, and colleagues. These form when a black hole belches out jets of hot plasma, which are then pushed out of the core by surrounding denser gas.

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Galaxies and galaxy clusters
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Professor Richard Bower of Durham University discusses galaxies and galaxy clusters whilst at the European Week of Astronomy and Space Science with Astronomy Now editor Keith Cooper.


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RE: Galaxy clusters
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Galaxies evolve over time by changing shape, size, and colour. Understanding the evolutionary processes of galaxies is one of the most important issues in astronomy for which there are still fundamental problems to be solved. One straightforward approach is to study clusters of galaxies that contain several thousands to tens of thousands of galaxies. Galaxy Clusters are dense populations of galaxies, rich with hot intergalactic gas, accompanied by strong gravitational forces. These clusters are the best location to observe environmental effects in galactic evolution.
To resolve the effect of the environment on the evolution of galaxies, a team of researchers from the National Astronomical Observatory of Japan and the University of Tokyo used Suprime-cam on the Subaru Telescope to observe the Coma Cluster of galaxies. The Coma Cluster contains over 1,000 galaxies and is fairly close to Earth at about 300 million light years away. The central portion of the Cluster is inhabited by mostly elliptical galaxies, both dwarf as well as giants.

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