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Register a new userThe eROSITA Final Equatorial-Depth Survey (eFEDS): Optical confirmation, redshifts, and properties of the cluster and group catalog
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The eROSITA Final Equatorial-Depth Survey (eFEDS), covering ~140 square degrees, was performed as part of the performance verification phase of the eROSITA telescope on board of the Russian-German satellite Spectrum-Roentgen-Gamma (SRG). In this paper we present the optical follow-up of 542 X-ray extent selected galaxy group and cluster candidates providing redshifts and cluster confirmation for the full sample. We use optical imaging data from the Hyper Suprime-Cam Subaru Strategic Program and from the Legacy Survey to run the cluster redshift and confirmation tool MCMF as well as the optical cluster finder CAMIRA at the location of the X-ray candidates. While providing redshift estimates for all 542 candidates, we construct an optically confirmed sample of 477 clusters and groups with a residual contamination of 6%. Of these, 470 (98.5%) are confirmed using MCMF and 7 systems are added through cross matching with spectroscopic group catalogs. Using observable to observable scaling and the applied confirmation threshold, we predict 8 +/- 2 real systems have been excluded with the MCMF cut required to build this low contamination sample. This number is in good agreement with the 7 systems recovered through cross matching. Thus, we expect those 477 systems to include >99% of all true clusters in the candidate list. Using an MCMF independent method, we confirm the catalog contamination of the confirmed subsample to be 6 +/- 3% and find 17 +/- 3% contamination for the full X-ray sample. The estimated contamination of the fulls sample is in agreement with MCMF dependent estimate of ~17% and the expectation from dedicated X-ray simulations of ~20%. We further present a sample of optically selected merging cluster candidates.
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The eROSITA Final Equatorial-Depth Survey has been carried out during the PV phase of the SRG/eROSITA telescope and completed in November 2019. This survey is designed to provide the first eROSITA-selected sample of galaxy clusters and groups and to test the predictions for the all-sky survey in the context of cosmological studies with clusters. In the 140 deg$^2$ area covered by eFEDS, 542 candidate clusters and groups are detected as extended X-ray sources, down to a flux of $sim10^{-14} $erg/s/cm$^2$ in the soft band (0.5-2 keV) within 1. In this work, we provide the catalog of candidate galaxy clusters and groups in eFEDS. We perform imaging and spectral analysis on the eFEDS clusters with eROSITA X-ray data, and study the properties of the sample. The clusters are distributed in the redshift range [0.01, 1.3], with the median redshift at 0.35. We obtain the ICM temperature measurement with $>2sigma$ c.l. for $sim$1/5 (102/542) of the sample. The average temperature of these clusters is $sim$2 keV. Radial profiles of flux, luminosity, electron density, and gas mass are measured from the precise modeling of the imaging data. The selection function, the purity and completeness of the catalog are examined and discussed in detail. The contamination fraction is $sim1/5$ in this sample, dominated by misidentified point sources. The X-ray Luminosity Function of the clusters agrees well with the results obtained from other recent X-ray surveys. We also find 19 supercluster candidates in eFEDS, most of which are located at redshifts between 0.1 and 0.5. The eFEDS cluster and group catalog provides a benchmark proof-of-concept for the eROSITA All-Sky Survey extended source detection and characterization. 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.
Context. The eROSITA X-ray telescope onboard the Spectrum-Roentgen-Gamma (SRG) observatory combines a large field of view and collecting area in the energy range $sim$0.2 to $sim$8.0 keV with the capability to perform uniform scanning observations of large sky areas. Aims. SRG/eROSITA performed scanning observations of the $sim$140 square degrees eROSITA Final Equatorial Depth Survey (eFEDS) field as part of its performance verification phase. The observing time was chosen to slightly exceed the depth of equatorial fields after the completion of the eROSITA all-sky survey. We present a catalog of detected X-ray sources in the eFEDS field providing source positions and extent information, as well as fluxes in multiple energy bands and document a suite of tools and procedures developed for eROSITA data processing and analysis, validated and optimized by the eFEDS work. Methods. A multi-stage source detection procedure was optimized and calibrated by performing realistic simulations of the eROSITA eFEDS observations. We cross-matched the eROSITA eFEDS source catalog with previous XMM-ATLAS observations, confirming excellent agreement of the eROSITA and XMM-ATLAS source fluxes. Result. We present a primary catalog of 27910 X-ray sources, including 542 with significant spatial extent, detected in the 0.2-2.3 keV energy range with detection likelihoods $ge 6$, corresponding to a point source flux limit of $approx 7 times 10^{-15}$ erg/cm$^2$/s in the 0.5-2.0 keV energy band. A supplementary catalog contains 4774 low-significance source candidates with detection likelihoods between 5 and 6. In addition, a hard band sample of 246 sources detected in the energy range 2.3-5.0 keV above a detection likelihood of 10 is provided. A description of the dedicated data analysis software, calibration database and standard calibrated data products is provided in appendix.
During the performance verification phase of the SRG/eROSITA telescope, the eROSITA Final Equatorial-Depth Survey (eFEDS) has been carried out. It covers a 140 deg$^2$ field located at 126$^circ <$ R.A. $< 146^circ$ and -3$^circ <$ Dec. $< +6^circ$ with a nominal exposure over the field of 2.2 ks. 542 candidate clusters were detected in this field, down to a flux limit $F_X sim 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 0.5-2 keV band. In order to understand radio-mode feedback in galaxy clusters, we study the radio emission of brightest cluster galaxies of eFEDS clusters, and we relate it to the X-ray properties of the host cluster. Using LOFAR we identify 227 radio galaxies hosted in the BCGs of the 542 galaxy clusters and groups detected in eFEDS. We treat non-detections as radio upper limits. We analyse the properties of radio galaxies, such as redshift and luminosity distribution, offset from the cluster centre, largest linear size and radio power. We study their relation to the intracluster medium of the host cluster. We perform statistical tests to deal with upper limits on the radio luminosities. BCGs with radio-loud AGN are more likely to lie close to the cluster centre than radio-quiet BCGs. There is a clear relation between the clusters X-ray luminosity and the radio power of the BCG. Statistical tests indicate that this correlation is not produced by selection effects in the radio band. We see no apparent link between largest linear size of the radio galaxy and central density of the host cluster. Converting the radio luminosity to kinetic luminosity, we find that radiative losses of the intracluster medium are in an overall balance with the heating provided by the central AGN. Finally, we tentatively classify our objects into disturbed and relaxed, and we show that the link between the AGN and the ICM apparently holds regardless of the dynamical state of the cluster.
Understanding the cluster population of clusters of galaxies is of the utmost importance for using cluster samples in both astrophysical and cosmological studies. We present an in-depth analysis of the X-ray morphological parameters of the galaxy clusters and groups detected in the eROSITA Final Equatorial-Depth Survey (eFEDS). We study the eROSITA X-ray imaging data for a sample of 325 clusters and groups that are significantly detected in the eFEDS field. We characterize their dynamical properties by measuring a number of dynamical estimators: concentration, central density, cuspiness, ellipticity, power-ratios, photon asymmetry, and Gini coefficient. The galaxy clusters and groups detected in eFEDS, covering a luminosity range of more than three orders of magnitude and large redshift range out to 1.2 provide an ideal sample for studying the redshift and luminosity evolution of the morphological parameters and characterization of the underlying dynamical state of the sample. Based on these measurements we construct a new dynamical indicator, relaxation score, for all the clusters in the sample. We find no evidence for bimodality in the distribution of morphological parameters of our clusters, rather we observe a smooth transition from the cool-core to non-cool-core and from relaxed to disturbed states. A significant evolution in redshift and luminosity is also observed in the morphological parameters examined in this study after carefully taking into account the selection effects. We determine that our eFEDS-selected cluster sample, differently than ROSAT-based cluster samples, is not biased toward relaxed clusters, but contains a similar fraction of disturbed as SZ surveys.
Context: After the successful launch of the Spectrum-Roentgen-Gamma (SRG) mission in July 2019, eROSITA, the soft X-ray instrument aboard SRG, performed scanning observations of a large contiguous field, namely the eROSITA Final Equatorial Depth Survey (eFEDS), ahead of the planned four-year all-sky survey. eFEDS yielded a large sample of X-ray sources with very rich multi-band photometric and spectroscopic coverage. Aims: We present here the eFEDS Active Galactic Nuclei (AGN) catalog and the eROSITA X-ray spectral properties of the eFEDS sources. Methods: Using a Bayesian method, we perform a systematic X-ray spectral analysis for all eFEDS sources. The appropriate model is chosen based on the source classification and the spectral quality, and, in the case of AGN, including the possibility of intrinsic (rest-frame) absorption and/or soft excess emission. Hierarchical Bayesian modeling (HBM) is used to estimate the spectral parameter distribution of the sample. Results: X-ray spectral properties are presented for all eFEDS X-ray sources. There are 21952 candidate AGN, which comprise 79% of the eFEDS sample. Despite a large number of faint sources with low photon counts, our spectral fitting provides meaningful measurements of fluxes, luminosities, and spectral shapes for a majority of the sources. This AGN catalog is dominated by X-ray unobscured sources, with an obscured (logNH>21.5) fraction of 10% derived by HBM. The power-law slope of the catalog can be described by a Gaussian distribution of 1.94+-0.22. Above a photon counts threshold of 500, nine out of 50 AGN have soft excess detected. For the sources with blue UV to optical color (type-I AGN), the X-ray emission is well correlated with the UV emission with the usual anti-correlation between the X-ray to UV spectral slope {alpha}_{OX} and the UV luminosity.
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