Abstract
Context. Studies of galaxy clusters provide stringent constraints on models of structure formation. Provided that selection effects are under control, large X-ray surveys are well suited to derive cosmological parameters, in particular those governing the dark energy equation of state. Aims. We forecast the capabilities of the all-sky eROSITA (extended ROentgen Survey with an Imaging Telescope Array) survey to be achieved by the early 2020s. We bring special attention to modelling the entire chain from photon emission to source detection and cataloguing. Methods. The selection function of galaxy clusters for the upcoming eROSITA mission is investigated by means of extensive and dedicated Monte-Carlo simulations. Employing a combination of accurate instrument characterisation and a state-of-the-art source detection technique, we determine a cluster detection efficiency based on the cluster fluxes and sizes. Results. Using this eROSITA cluster selection function, we find that eROSITA will detect a total of approximately 105 clusters in the extra-galactic sky. This number of clusters will allow eROSITA to put stringent constraints on cosmological models. We show that incomplete assumptions on selection effects, such as neglecting the distribution of cluster sizes, induce a bias in the derived value of cosmological parameters. Conclusions. Synthetic simulations of the eROSITA sky capture the essential characteristics impacting the next-generation galaxy cluster surveys and they highlight parameters requiring tight monitoring in order to avoid biases in cosmological analyses.
Highlights
Clusters of galaxies are the most massive matter halos
This algorithm scans an X-ray image with a sliding square box, and if the signal-tonoise ratio (S/N) in the box is greater than a specified threshold value it is marked as a source candidate
In order to increase the sensitivity for large extended sources, this procedure is repeated for 2 × 2 and 4 × 4 rebinned images corresponding to kernels with rc = 30 and rc = 60 arcsec, respectively
Summary
Clusters of galaxies are the most massive matter halos They formed last in the history of the Universe by a hierarchical growth of structures in the Hubble expansion flow. The identification and study of the different components of galaxy clusters (dark matter halo, intracluster medium, galaxies, and relativistic particles) require the use of several different observational techniques. Among such techniques, X-ray observations stand out, since clusters of galaxies are the most luminous extended sources in the extra-galactic X-ray sky, and are detectable in large surveys. It has been established that observational selection effects play a crucial role and must be controlled accurately when pursuing the goal of precision cosmology (e.g. Vikhlinin et al 2009; Mantz et al 2010b; Allen et al 2011; Pacaud et al 2016)
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