Cosmology and astrophysics from relaxed galaxy clusters – I. Sample selection

Abstract

This is the first in a series of papers studying the astrophysics and cosmology of massive, dynamically relaxed galaxy clusters. Here we present a new, automated method for identifying relaxed clusters based on their morphologies in X-ray imaging data. While broadly similar to others in the literature, the morphological quantities that we measure are specifically designed to provide a fair basis for comparison across a range of data quality and cluster redshifts, to be robust against missing data due to point-source masks and gaps between detectors, and to avoid strong assumptions about the cosmological background and cluster masses. Based on three morphological indicators -Symmetry, Peakiness and Alignment -we develop the SPA criterion for relaxation. This analysis was applied to a large sample of cluster observations from the Chandra and ROSAT archives. Of the 361 clusters which received the SPA treatment, 57 (16 per cent) were subsequently found to be relaxed according to our criterion. We compare our measurements to similar estimators in the literature, as well as projected ellipticity and other image measures, and comment on trends in the relaxed cluster fraction with redshift, temperature, and survey selection method. Code implementing our morphological analysis will be made available on the web. 1

The Remarkable Similarity of Massive Galaxy Clusters from z ∼ 0 to z ∼ 1.9

“…This appears to contradict the prediction from simulations that the merger rate is a strong function of redshift (see, e.g., but is consistent with other groups that have studied the evolution of cluster morphology (e.g., Mantz et al 2015;Nurgaliev et al 2017). For the most massive halos ( >  M M 10 14 ), found that the rate of major (…”

“…The X-ray morphology of a galaxy cluster is commonly used as a probe of the cluster's dynamical state (e.g., Mohr et al 1995;Schuecker et al 2001;Weißmann et al 2013;Mantz et al 2015). Nurgaliev et al (2017) demonstrated that the measured value of a phot , which we use in this work to quantify morphology, is significantly elevated during a major merger for ∼1-2 Gyr, based on hydrodynamic simulations of 26 major (M 1 /M 2 >0.5) cluster mergers.…”

“…The eighthigh-z clusters span the full range of morphologies, occupying all parts of this parameter space. 17 We use the "symmetry" (S) parameter from Mantz et al (2015) to identify disturbed clusters. Using overlapping clusters from the analyses of Mantz et al (2015) and Nurgaliev et al (2017), we find that S<0.6 is roughly equivalent to > a 0.5 phot .…”

“…17 We use the "symmetry" (S) parameter from Mantz et al (2015) to identify disturbed clusters. Using overlapping clusters from the analyses of Mantz et al (2015) and Nurgaliev et al (2017), we find that S<0.6 is roughly equivalent to > a 0.5 phot . times higher at z∼1 compared to that atz∼0 , while the relaxation time is ∼2 times shorter (H(z) is twice as large) over the same redshift interval.…”

Cosmology and astrophysics from relaxed galaxy clusters – V. Consistency with cold dark matter structure formation

“…The selection of the sample of massive, relaxed clusters employed here is detailed in the first paper in this series (Mantz et al 2015a; hereafter Paper I). Papers II, III and IV (Mantz et al 2014(Mantz et al , 2016Applegate et al 2016) employ Chandra data for this sample to respectively constrain cosmological parameters, through gas-mass fraction measurements; scaling relations and thermodynamics of the ICM; and the average bias of the X-ray mass determinations.…”