The SRG/eROSITA all-sky survey: The morphologies of clusters of galaxies

The first SRG/eROSITA all-sky X-ray survey, eRASS1, resulted in a catalogue of over 12 000 optically confirmed galaxy groups and clusters in the western Galactic hemisphere. Using the eROSITA images of these objects, we measured and studied their morphological properties, including their concentration, central density and slope, ellipticity, power ratios, photon asymmetry, centroid shift, and Gini coefficient. We also introduced new forward-modelled parameters that take account of the instrument point spread function (PSF), namely, slosh, which measures how asymmetric the surface brightness distribution is, and multipole magnitudes, which are analogues to power ratios. Using simulations, we found that some non-forward-modelled parameters are strongly biased due to PSF and data quality. When using Chandra and previous results from XMM-Newton , we found similar values of concentration and central density compared to our results for the same clusters. The population as a whole has log concentrations that are typically around 0.3 dex larger than samples selected from the South Pole Telescope or Planck and the deeper eFEDS sample. The exposure time, detection likelihood threshold, extension likelihood threshold, and number of counts affect the concentration distribution but generally not enough to reduce the concentration to match the other samples. The concentration of clusters in the survey strongly affects whether they are detected as a function of redshift and luminosity. We introduced a combined disturbance score based on a Gaussian mixture model fit to several of the parameters. For brighter clusters, around one-fourth of the objects are classified as disturbed using this score, which may be due to our sensitivity to concentrated objects.