In the first three months of its operations (October-November 2019), eROSITA scanned a 140 deg2 area along the Equatorial strip, i.e. the eROSITA Final Equatorial Depth Survey (eFEDS)—the goals of these observations were to test the source detection process, pre-launch predictions on cluster number counts, and selection modeling. In the eFEDS area, we find 479 confirmed clusters and groups of galaxies that are detected as extended X-ray sources with the source detection algorithm. The eFEDS galaxy cluster and groups with a median redshift of 0.32 confirm the excellent performance of eROSITA for cluster science and expect no significant deviations from our pre-launch expectations for the final all-sky survey. This catalog is published in A&A (Liu A., Bulbul E., et al., 2022, A&A, 661, A2), and the data, catalogs, and publication are publicly available. This work was highlighted in the eFEDS data release press coverage (
https://www.mpe.mpg.de/7650552/news20210628?c=260760(s’ouvre dans une nouvelle fenêtre)).
To test the selection biases claimed for X-ray surveys, we further explored the eFEDS sample to investigate the selection effects of eROSITA. We studied the eROSITA X-ray imaging data for a sample of 325 clusters and groups significantly detected in the eFEDS field. We characterized their dynamical properties by measuring several dynamical estimators: concentration, central density, cuspiness, centroid shift, ellipticity, power ratios, photon asymmetry, and the Gini coefficient. We find no evidence for a bimodality in the distribution of the morphological parameters of our clusters. We instead observe a smooth transition from the cool core to the non-cool core and from relaxed to disturbed states, with a preference for skewed distributions or log-normal distributions. This work is also published in A&A (Ghirardini V., Bahar Y. E., Bulbul E., et al., 2022, A&A, 661, A12.)
One essential step in constructing the cluster mass function is to model the scaling relations between the X-ray observable and the weak lensing mass. We model the X-ray observable-to-mass-and-redshift relations using the three-year (S19A) weak-lensing data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey, including the rest-frame soft-band and bolometric luminosity (LX and Lb), the emission-weighted temperature, the mass of intra-cluster medium, and the mass proxy (Bahar Y. E., Bulbul E., et al., 2022, A&A, 661, A7; Chiu I. N., Ghirardini V., et al., 2022, A&A, 661, A11). My group has performed the X-ray observable measurements of the eFEDS-detected clusters in the field and published the scaling relations.
One of the major discoveries in the eFEDS field was the detection of eROSITA’s first superclusters in the eFEDS field (Ghirardini V., Bulbul E., et al., 2021, A&A, 647, A4.) Superclusters constitute a significant fraction of the cosmic web and are the largest non-virialized objects in the Universe. After the first discovery, we performed an independent search in the eFEDS field and located 19 superclusters (Liu A., Bulbul E., et al., 2022, A&A, 661, A2). This work showed for the first time the power of eROSITA in detecting and mapping the large-scale structure and was highlighted in a press release (
https://phys.org/news/2020-12-supercluster-astronomers.html(s’ouvre dans une nouvelle fenêtre)). The supercluster detection algorithm developed on the eFEDS field is used on the first All-Sky Survey to locate all massive superclusters above our flux limit (Liu, A., Bulbul, E., et al., 2023)
Additionally, we applied our source detection and X-ray data reduction routines. We tested on the eFEDS field using the ERC funds on the first eROSITA All-Sky Survey. We identified 12,247 optically confirmed galaxy groups and clusters detected in the 0.2-2.3 keV band as extended X-ray sources in a 13,116 deg2 region in the western Galactic half of the sky, which eROSITA surveyed in its first six months of operation. This catalog represents the largest ICM-selected galaxy cluster and group catalog to date (Bulbul et al. 2024, Kluge et al. 2022). In the third review period, we have successfully applied the developed methods on eFEDS to the first eROSITA All-Sky Survey, realizing the first constraints on the cosmological parameters, normalization of the density fluctuations, the energy density of total matter, and the total mass in neutrinos. We have been published as a set of 13 papers (Ghirardini et al. 2024, Artis et al. 2024, Artis et al. 2025, Seppi et al. 2024, Grandis et al. 2024, Kleinebreil et al. 2024, Zelmer et al. 2025, Clerc et al. 2024, Chiu et al. 2025, Okabe et al. 2025, Bahar et al. 2024, Liu et al. 2024).
In the current period, we worked on analyzing and preparing the deepest eROSITA data, final preparations for the final cosmology results, which are expected to reach the CMB precision.
