The First SRG/eROSITA All-Sky Survey. Characterization of clusters of galaxies misclassified in the eRASS1 point source catalog

TL;DR

This paper investigates galaxy clusters misclassified as point sources in the first eROSITA All-Sky Survey (eRASS1) by applying optical follow-up and red-sequence identification at X-ray point-source positions. Using eROMaPPer to detect red-sequence overdensities and NWAY counterparts to link X-ray detections with optical/IR sources, the authors assemble a high-purity catalog of 8347 misclassified clusters, of which 5819 are new discoveries, spanning $0.05 < z \lesssim 1.1$ and revealing substantial overlap with existing catalogs while also uncovering a large population of high-redshift or compact-core systems that would be missed by extent-based selections. They introduce a six-class scheme (0–5) based on counterpart reliability, Galactic contamination, and redshift consistency to characterize these clusters and their X-ray emission, including detailed cross-matches with optical, X-ray, and SZ surveys. The work provides insights into selection biases in shallow X-ray surveys and demonstrates the potential for discovering unusual clusters (e.g., strong cool-core systems or AGN-embedded clusters) that will be detectable as extended sources in deeper eROSITA data. The resulting catalog, with X-ray and optical properties and class flags, enables targeted follow-up studies of AGN feedback, cluster assembly, and absorption studies of the ICM in the eROSITA era.

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 12000 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 their vicinity. 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 $0.05 < z \lesssim 1.1$. 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.

Figures (16)

• Figure 1: Illustration of the data flow discussed in Sect. \ref{['ssec:data_intro_cat_cleaning']} for assembling the cleaned and merged sample.
• Figure 2: Distribution of the maximally shared member fraction $f_{\rm s, max}$ (Eq. \ref{['eq:maximally_shared_member_fraction']}) as a function of redshift for 9518.0 misclassified clusters that share at least one member. $f_{\rm s, max}$ is calculated w.r.t. the joint member sample of other misclassified clusters and the extent-selected clusters presented by Kluge2024. The histograms in the top and right panels show projections of the distributions. The dashed red line indicates the threshold at $f_{\rm s, max}=0.7$ that is adopted to mark candidates as part of associations with others.
• Figure 3: Illustration of the transitive association process. The numbers of members and shared members are arbitrarily chosen. The labels on the arrows indicate the number of shared members between the respective candidates; their colors denote whether their shared numbers would lead to the two candidates being associated (green) or not (orange).
• Figure 4: Distribution of the number of cluster candidates in each transitive association sharing members above the threshold. The total height of the green bars and the numbers annotated show the total amount of associations that contain $N_{\rm candidate}$. The yellow bars display the number of associations containing $N_{\rm candidate}$ where none of the included candidates in the association is an extent-selected cluster. The dashed line shows the fraction of these associations as a function of the total amount for each $N_{\rm candidate}$. For better visibility, we omit the 41.0 associations where $N_{\rm candidate}$ exceeded 20.0, most notably one association containing 50.0 overlapping candidates.
• Figure 5: Illustration of the classification process for the cluster classes introduced in Sect. \ref{['ssec:data_intro_cluster_class_definition']}. We abbreviate the counterpart associated via NWAY to CTP for better readability. The decision points are described in detail in Sect. \ref{['sssec:classification_parameters']}.