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
We present results from the analysis of a mosaic of 13 XMM–Newton pointings covering the Virgo Cluster from its centre northwards out to a radius r∼ 1.2 Mpc (∼45), reaching the virial radius and beyond. This is the first time that the properties of a modestly sized (Mvir∼ 1.4 × 1014 M⊙, kT∼ 2.3 keV), dynamically young cluster have been studied out to the virial radius. The density profile of the cluster can be described by a surprisingly shallow power‐law ne∝r−β with index β= 1.21 ± 0.12. In the radial range of 0.3rvir < r < rvir, the best‐fitting temperature drops by roughly 60 per cent. Within a radius r < 450 kpc, the entropy profile has an approximate power law form K∝r1.1, as expected for gravitationally collapsed gas in hydrostatic equilibrium. Beyond r∼ 450 kpc, however, the temperature and metallicity drop abruptly, and the entropy profile becomes flatter, staying consistently below the expected value by a factor of 2–2.5. The most likely explanation for the unusually shallow density profile and the flattening of entropy at large radius is clumping in the ICM. Our data provide direct observational evidence that the ICM is enriched by metals all the way to r200 to at least Z= 0.1 Z⊙.
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