The first SRG/eROSITA All-Sky Survey

Characterization of clusters of galaxies misclassified in the eRASS1 point source catalog

¹ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
² Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
³ IRAP, Université de Toulouse, CNRS, UPS, CNES, 31028 Toulouse, France
⁴ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, via Piero Gobetti 93/3, 40129 Bologna, Italy
⁵ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁶ Arnold Sommerfeld Center for Theoretical Physics, LMU Munich, Theresienstr. 37, 80333 München, Germany
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

^★ Corresponding author: fbalzer@mpe.mpg.de

Received: 28 January 2025
Accepted: 15 May 2025

Abstract

The detection of the extended X-ray emission of the intracluster medium by the first SRG/eROSITA All-Sky Survey (eRASS1), combined with optical and near-infrared follow-up, resulted in the identification of more than 12 000 galaxy clusters, yielding precise constraints on cosmological parameters. However, some clusters of galaxies can be misclassified as point sources by eROSITA’s source detection algorithm due to the interplay between the point spread function, the shallow depth of the survey, compact (cool core) X-ray emission, and bright active galactic nuclei hosted in their centers or nearby. To identify such misclassified galaxy clusters and groups, we apply optical follow-up to the eRASS1 X-ray point sources analogously to the treatment of the extent-selected catalog. After rigorous filtering to ensure purity, we find a total of 8347 clusters of galaxies, of which 5819 are novel detections, in a redshift range 0.05 < z ≲ 1.1. This corresponds to a 70% discovery rate, a fraction similar to that of the extent-selected sample. To facilitate finding new exceptional clusters such as the Phoenix cluster (which is recovered in our sample), we divide the clusters into five classes based on the optical properties of likely single-source counterparts to the X-ray emission. We further investigate potential biases in our selection process by analyzing the optical and X-ray data. With this work, we provide a catalog of galaxy clusters and groups in the eRASS1 point source catalog, including their optical and X-ray properties along with a meaningful classification.