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

Abstract

This is the first in a series of papers studying the astrophysics and\ncosmology of massive, dynamically relaxed galaxy clusters. Here we present a\nnew, automated method for identifying relaxed clusters based on their\nmorphologies in X-ray imaging data. While broadly similar to others in the\nliterature, the morphological quantities that we measure are specifically\ndesigned to provide a fair basis for comparison across a range of data quality\nand cluster redshifts, to be robust against missing data due to point-source\nmasks and gaps between detectors, and to avoid strong assumptions about the\ncosmological background and cluster masses. Based on three morphological\nindicators - Symmetry, Peakiness and Alignment - we develop the SPA criterion\nfor relaxation. This analysis was applied to a large sample of cluster\nobservations from the Chandra and ROSAT archives. Of the 361 clusters which\nreceived the SPA treatment, 57 (16 per cent) were subsequently found to be\nrelaxed according to our criterion. We compare our measurements to similar\nestimators in the literature, as well as projected ellipticity and other image\nmeasures, and comment on trends in the relaxed cluster fraction with redshift,\ntemperature, and survey selection method. Code implementing our morphological\nanalysis will be made available on the web.\n