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eFEDS

eFEDS Cluster Catalog

The eFEDS cluster and group catalog at the final eRASS equatorial depth provide a benchmark proof of concept for the eROSITA All-Sky Survey extended source detection and characterization. The eFEDS cluster sample includes 477 optically confirmed low-mass clusters and groups in the redshift range of 0.01 to 1.3. In the movie below, each dot represents a cluster or group, with its color indicating cluster redshift, and its size is proportional to the size of the cluster. eROSITA is also powerful in mapping the large-scale structure. In the eFEDS field, we located 19 superclusters.  The article describing the catalog and the X-ray properties of galaxy groups, galaxy clusters, and superclusters is published in Liu, A. et al. (2022). The catalog itself is available to download on this page. In the same work, we compare the X-ray luminosity function, i.e., the number of clusters per unit volume per unit luminosity interval,  of the eFEDS clusters with the results in the literature. We find that the cluster number sounds and their luminosity measurements are consistent with the previous results taking into account the selection effects. We 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.

Morphological Properties of eFEDS Clusters

Morphological parameters are the estimators for the dynamical state of clusters of galaxies. Understanding the underlying population of clusters of galaxies in surveys is of the utmost importance when these samples are used in astrophysical and cosmological studies. In Ghirardini et al. (2022), we present an in-depth analysis of the X-ray morphological parameters. (concentration, central density, cuspiness, centroid shift, ellipticity, power ratios, photon asymmetry, and the Gini

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coefficient) of the galaxy clusters and groups detected in eFEDS. We find no evidence for 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. A significant evolution in redshift and luminosity is also observed in the morphological parameters we examined after taking the selection effects into account.

Scaling Relations

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Scaling relations link the physical properties of clusters at cosmic scales. They are used to probe the evolution of large-scale structure, estimate observables of clusters, and constrain cosmological parameters through cluster counts. In Bahar et al. (2022) and Chiu et al. (2022), we investigate the scaling laws between X-ray observables, e.g., luminosity, the temperature of the intra-cluster medium,  and cluster mass, by accounting for the selection biases. We find from the self-similar scenario that it assumes gravitational collapse. These results indicate that the non-gravitational effects play an important role in shaping the observed physical state of clusters. This work extends the scaling relations to the low-mass, low-luminosity galaxy cluster, and group regime, demonstrating the ability of eROSITA to measure emission from the intracluster medium out to the overdensity radius, R500, with survey-depth exposures and constrain the scaling relations in a wide mass-luminosity-redshift range.

Estimating Cluster Masses with Neural Networks

Cosmology analyses through cluster abundances heavily rely on the availability of external mass measurements. In Krippendorf et al. (2023) we develop a neural network based pipeline to estimate masses of galaxy clusters from X-ra images. Our neural networks are trained using supervised learning on simulations of the eFEDS field.  Using this method, we are able to provide for the first time neural network mass estimation for the observed eFEDS cluster sample and we find consistent performance with masses measured via weak lensing observations. When compared to simulated data, we observe a reduced scatter with respect to luminosity and count-rate based scaling relations. The figure shows the comparisons between neural network inferred mass measurements with those measured with weak lensing.

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A Deeper Study of Selection Effects

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The common area between the eFEDS field and the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) S19A survey, is an ideal region to compare the properties of galaxy clusters selected in X-ray and weak lensing.  In the work lead by Ramos-Ceja et al. (2022), we investigate the effects of selection methods in the galaxy cluster detection by comparing the X-ray selected, eFEDS, and the shear-selected, HSC-SSP S19A, galaxy cluster samples. Our results indicate that the scaling relation between X-ray luminosity and true cluster mass of the shear-selected cluster sample is consistent with the eFEDS sample. We find that X-ray selected cluster samples are complete and can be used as an accurate cosmological probe. The figure on the left shows the galaxy overdensity map overlaid with X-ray contours in red of one of the commonly detected cluster.

Clusters of Galaxies in Disguise

Because of the sizeable point-spread function of eROSITA, high-redshift clusters of galaxies or compact nearby groups hosting bright active galactic nuclei (AGN) can be misclassified as point sources by the source detection algorithms. In Bulbul et al. (2022), we indeitified a total of 346 galaxy clusters and groups in the redshift range of 0.1 < z < 1.3 in the eFEDS point source catalog. This study help us understand our selection process and assess the completeness of the eROSITA extent-selected samples. The method we developed is used to identify high-redshift clusters, AGN-dominated groups, and low-mass clusters that are misclassified in the future eROSITA all-sky survey point source catalogs. Figure on the right shows one of the spectacular galaxy groups whose extended emission is washed out by the central AGN discovered in this study.

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The Discovery of the First Supercluster of eROSITA

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Superclusters are the overdense regions of dark matter and baryons in the cosmic web. Superclusters are composed of a number of galaxy clusters and groups connected to each other through cosmic filaments. In Ghiradini et al. (2021), we discoevered the first supercluster detected by eROSITA in the eFEDS field. This supercluster is composed of eight galaxy groups and clusters at the redshift of z = 0.36. This study demostrates eROSITA's potential to map the large scale structure in the Universe.  In a more intense search, we found a total of 19 superclsuters alone in the eFEDS field (see Liu et al. 2022). Our forecasts show that we will be able to detect about 450 superclusters when the survey is completed. These results captured a great press attention. The image on the left shows the members of the supercluster, the figure inset is the galaxy density map obtained from the Hypersupreme Camera data.

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