Forecasts for cosmological measurements based on the angular power spectra of AGN and clusters of galaxies in the SRG/eROSITA all-sky survey

Abstract

Context. The eROSITA X-ray telescope aboard the Spectrum-Roentgen-Gamma (SRG) orbital observatory, in the course of its all-sky survey, is expected to detect about three million active galactic nuclei (AGNs) and approximately one hundred thousand clusters and groups of galaxies. Such a sample, clean and uniform, complemented with redshift information, will open a new window into the studies of the large-scale structure (LSS) of the Universe and the determination of its cosmological parameters. Aims. The purpose of this work is to assess the prospects of cosmological measurements with the eROSITA sample of AGNs and clusters of galaxies. We assumed the availability of photometric redshift measurements for eROSITA sources and explored the impact of their quality on our forecasts. Methods. As the LSS probe, we use the redshift-resolved angular power spectrum of the density fluctuations of objects. We employed a Fisher-matrix formalism and assumed flat ΛCDM cosmology to forecast the constraining power of eROSITA samples of AGNs and clusters of galaxies. We computed the LSS-relevant characteristics of AGNs and clusters in the framework of the halo model and their X-ray luminosity functions. As the baseline scenario, we considered the full four-year all-sky survey and investigated the impact of reducing the survey length to two years. Results. We find that the accuracy of photometric redshift estimates has a more profound effect on cosmological measurements than the fraction of catastrophic errors. Under realistic assumptions about the photometric redshift quality, the marginalised errors on the cosmological parameters achieve 1 − 10% accuracy depending on the cosmological priors used from other experiments. The statistical significance of Baryon acoustic oscillation detection in angular power spectra of AGNs and clusters of galaxies considered individually achieves 5 − 6σ. Our results demonstrate that the eROSITA sample of AGNs and clusters of galaxies used in combination with currently available photometric redshift estimates will provide cosmological constraints on a par with dedicated optical LSS surveys.