Toward a characterization of X-ray galaxy clusters for cosmology

Abstract

Context. In the framework of the hierarchical model the intra-cluster medium properties of galaxy clusters are tightly linked to structure formation, which makes X-ray surveys well suited for cosmological studies. To constrain cosmological parameters accurately by use of galaxy clusters in current and future X-ray surveys, a better understanding of selection effects related to the detection method of clusters is needed. Aims. We aim at a better understanding of the morphology of galaxy clusters to include corrections between the different core types and covariances with X-ray luminosities in selection functions. In particular we stress the morphological deviations between a newly described surface brightness profile characterization and a commonly used single β-model. Methods. We investigate a novel approach to describe surface brightness profiles, where the excess cool-core emission in the centres of the galaxy clusters is modelled using wavelet decomposition. Morphological parameters and the residuals are compared to classical single β-models, fitted to the overall surface brightness profiles. Results. Using single β-models to describe the ensemble of overall surface brightness profiles leads on average to a non-zero bias (0.032 ± 0.003) in the outer part of the clusters, i.e. a ∼ 3% systematic difference in the surface brightness at large radii. In addition β-models show a general trend towards underestimating the flux in the outskirts for smaller core radii. Fixing the β parameter to 2/3 doubles the bias and increases the residuals from a single β-model up to more than 40%. Modelling the core region in the fitting procedure reduces the impact of these two effects significantly. In addition, we find a positive scaling between shape parameters and temperature, as well as a negative correlation (∼ −0.4) between extent and luminosity. Conclusions. We demonstrate the caveats in modelling galaxy clusters with single β-models and recommend the usage of them with caution, especially when not taking the systematics into account that arise using them. Our non-parametric analysis of the self-similar scaled emission measure profiles indicates no systematic core-type differences of median profiles in the galaxy clusters outskirts. Article number, page 1 of 41 arXiv:1907.03806v1 [astro-ph.CO] 8 Jul 2019 A&A proofs: manuscript no. AA_2019_35124Outside the core regions, scaled radial profiles (e.g. temperature, pressure or entropy profiles) of galaxy clusters show a socalled "self-similar" behaviour (e.g. Zhang et al. 2007; Ghirardini et al. 2018). It is believed that this is the result of a similar formation process of galaxy clusters, namely that tiny density perturbations in the early universe are amplified by gravitational instabilities and grow hierarchically, yielding the large scale structure observed today. Galaxy clusters are then believed to correspond to the densest regions of the large scale structure. This formation history motivated the theoretical consideration of the self-similar model (e.g. Kaiser 1986), whe...

A tight correlation between the enclosed gravitational mass and hot gas mass in galaxy clusters at intermediate radii

“…This problem is more severe for strong cool-core clusters. However, for noncool-core or weak-cool-core clusters, the accuracy of the flux in the outskirts stays at the ∼ 4% level for these clusters [45]. In our 64 sample clusters, we have only 12 cool-core clusters and only 4 of them are strong cool-core clusters.…”

“…Although using the single-β model may introduce some systematic errors, we have to use the single core radii r c as reference radii of our sample clusters for consistency (two core radii appeared in the double-β model for each cluster). Furthermore, recent studies show that the flux in the outskirts is systematically under-predicted by using a single-β model [45]. This problem is more severe for strong cool-core clusters.…”

Toward the low-scatter selection of X-ray clusters

“…Our source lists can be used with the ML fitting in its standard form and in the modified form, in which the core radii of the clusters are examined only at large radii. This avoids the influence of the cool cores on the estimate, as found by Käfer et al (2019).…”

“…This ensures that the apparent size scales with redshift, that is, clusters at higher redshift have a smaller angular extent. The relation between core and overdensity radii is calibrated on non-cool-core clusters at low redshift (Käfer et al 2019) and holds at high redshift, where the relative contribution of the cool core to the outer parts of the cluster becomes minor (McDonald et al 2013). The compactness of clusters at high redshift matters for the detection.…”

“…Determining the selection function is especially complex for extended X-ray sources because the detection probability and proper classification depend on their morphology, for example (Eckert et al 2011;Rossetti et al 2016Rossetti et al , 2017Andrade-Santos et al 2017;Lovisari et al 2017). The cluster outskirts (0.2-0.8 r 500 ) are found to evolve with redshift in a self-similar fashion (McDonald et al 2017;Käfer et al 2019) and exhibit low scatter (Ghirardini et al 2018;Käfer et al 2019). Therefore, cluster samples that are selected based on the properties of cluster outskirts will closely trace the selection by cluster mass and reduce the systematics of cluster use in cosmological studies.…”