Substructure of the galaxy clusters in the REXCESS sample: observed statistics and comparison to numerical simulations

📝 Abstract

We study the substructure statistics of a representative sample of galaxy clusters by means of two currently popular substructure characterisation methods, power ratios and centroid shifts. We use the 31 clusters from the REXCESS sample, compiled from the southern ROSAT All-Sky cluster survey REFLEX with a morphologically unbiased selection in X-ray luminosity and redshift, all of which have been reobserved with XMM-Newton. We investigate the uncertainties of the substructure parameters and examine the dependence of the results on projection effects, finding that the uncertainties of the parameters can be quite substantial. Thus while the quantification of the dynamical state of individual clusters with these parameters should be treated with extreme caution, these substructure measures provide powerful statistical tools to characterise trends of properties in large cluster samples. The centre shift parameter, w, is found to be more sensitive in general. For the REXCESS sample neither the occurence of substructure nor the presence of cool cores depends on cluster mass. There is a significant anti-correlation between the existence of substantial substructure and cool cores. The simulated clusters show on average larger substructure parameters than the observed clusters, a trend that is traced to the fact that cool regions are more pronounced in the simulated clusters, leading to stronger substructure measures in merging clusters and clusters with offset cores. Moreover, the frequency of cool regions is higher in the simulations than in the observations, implying that the description of the physical processes shaping cluster formation in the simulations requires further improvement.

💡 Analysis

We study the substructure statistics of a representative sample of galaxy clusters by means of two currently popular substructure characterisation methods, power ratios and centroid shifts. We use the 31 clusters from the REXCESS sample, compiled from the southern ROSAT All-Sky cluster survey REFLEX with a morphologically unbiased selection in X-ray luminosity and redshift, all of which have been reobserved with XMM-Newton. We investigate the uncertainties of the substructure parameters and examine the dependence of the results on projection effects, finding that the uncertainties of the parameters can be quite substantial. Thus while the quantification of the dynamical state of individual clusters with these parameters should be treated with extreme caution, these substructure measures provide powerful statistical tools to characterise trends of properties in large cluster samples. The centre shift parameter, w, is found to be more sensitive in general. For the REXCESS sample neither the occurence of substructure nor the presence of cool cores depends on cluster mass. There is a significant anti-correlation between the existence of substantial substructure and cool cores. The simulated clusters show on average larger substructure parameters than the observed clusters, a trend that is traced to the fact that cool regions are more pronounced in the simulated clusters, leading to stronger substructure measures in merging clusters and clusters with offset cores. Moreover, the frequency of cool regions is higher in the simulations than in the observations, implying that the description of the physical processes shaping cluster formation in the simulations requires further improvement.

📄 Content

arXiv:0912.4667v1 [astro-ph.CO] 23 Dec 2009 Astronomy & Astrophysics manuscript no. Boehringer c⃝ESO 2018 March 12, 2018 Substructure of the galaxy clusters in the REXCESS sample: observed statistics and comparison to numerical simulations H. B¨ohringer1, G.W. Pratt2,1, M. Arnaud2, S. Borgani3, J.H. Croston4, T.J. Ponman5, S. Ameglio3,6, R.F. Temple5 and K. Dolag7 1 Max-Planck-Institut f¨ur extraterrestrische Physik, D 85748 Garching, Germany, hxb@mpe.mpg.de 2 Laboratoire AIM, IRFU/Service d’Astrophysique - CEA/DSM - CNRS - Universit´e Paris Diderot, Bˆat. 709, CEA-Saclay, F-91191 Gif-sur-Yvette Cedex, France 3 Dipartimento di Astronomia dell’Universit´a di Trieste, via Tiepolo 11, I-34133 Trieste, Italy 4 School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK 5 School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 6 University of Southern California, Los Angeles, CA, USA 7 Max-Planck-Institut f¨ur Astrophysik, D 85748 Garching, Germany Received xxxx; Accepted yyyy ABSTRACT We study the substructure statistics of a representative sample of galaxy clusters by means of two currently popular substructure characterisation methods, power ratios and centroid shifts. We use the 31 clusters from the REXCESS sample, compiled from the southern ROSAT All-Sky cluster survey (REFLEX) with a morphologically unbiased selection in X-ray luminosity and redshift, all of which have been reobserved with XMM-Newton. The main goals of this work are to study the relationship between cluster morphology and other bulk properties, and the comparison of the morphology statistics between observations and numerical simulations. We investigate the uncertainties of the substructure parameters via newly-developed Monte Carlo methods, and examine the dependence of the results on projection effects (via the viewing angle of simulated clusters), finding that the uncertainties of the parameters can be quite substantial. Thus while the quantification of the dynamical state of individual clusters with these parameters should be treated with extreme caution, these substructure measures provide powerful statistical tools to characterise trends of properties in large cluster samples. The centre shift parameter, w, is found to be more sensitive in general and offers a larger dynamic range than the power ratios. For the REXCESS sample neither the occurence of substructure nor the presence of cool cores depends on cluster mass; however a weak correlation with X-ray luminosity is present, which is interpreted as selection effect. There is a significant anti-correlation between the existence of substantial substructure and cool cores. The simulated clusters show on average larger substructure parameters than the observed clusters, a trend that is traced to the fact that cool regions are more pronounced in the simulated clusters, leading to stronger substructure measures in merging clusters and clusters with offset cores. Moreover, the frequency of cool regions is higher in the simulations than in the observations, implying that the description of the physical processes shaping cluster formation in the simulations requires further improvement. Key words. X-rays: galaxies: clusters, Galaxies: clusters: Intergalactic medium, Cosmology: observations

1. Introduction There are two major far-reaching interests that motivate our un- derstanding the population of galaxy clusters. Firstly, they are ideal test objects to check the likelihood of a given cosmologi- cal model to describe our Universe; secondly, they are very im- portant probes of the astrophysical and chemical evolution of the baryonic component of the Universe (e.g. Rosati et al. 2002, Schuecker et al. 2003a,b, Voit 2005, Vikhlinin et al. 2003, 2009, Rozo et al. 2007, Henry et al. 2009, Mantz et al. 2008). A good understanding and observational characterisation of galaxy clus- ters is required to attain these goals. X-ray observations provide an essential window into the study of galaxy clusters, as the presence of X-ray radiation im- plies and traces a well developed gravitational well (e.g. Sarazin 1986, B¨ohringer 2008), offering the best starting point for the characterisation of the cluster mass and dynamical state. The temperature of the hot intracluster medium is related to the depth Send offprint requests to: H. B¨ohringer, hxb@mpe.mpg.de of the potential well, and its distribution is related to the dy- namical state of the system. In this study we use statistical mea- sures of substructure observed in the X-ray images, which pro- vide a projected view of the ICM structure, to obtain an impres- sion of the cluster’s dynamical state1. Ideally one would like to base such a study on a known relation between a substructure parameterization and a measure of the deviation from dynami- cal equilibrium of the cluster. While this could be an important aim for a future study based on simulations2, we pursue here the more qualitative goal

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