The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930 000 entries detected in the most sensitive 0.2–2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3–5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We present methods to identify and flag potential spurious sources in the catalogues, which we applied for this work, and we tested and validated the astrometric accuracy via cross-comparison with other X-ray and multi-wavelength catalogues. We show that the number counts of X-ray sources in eRASSl are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) ofF_{05–2 keV}> 5 × 10⁻¹⁴erg s⁻¹cm⁻², we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1–2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided.

The evolution of the cluster mass function traces the growth of linear density perturbations, providing valuable insights into the growth of structures, the nature of dark matter, and the cosmological parameters governing the Universe. The primary science goal of eROSITA, on board the Spectrum Roentgen Gamma (SRG) mission, is to constrain cosmology through the evolution of the cluster mass function. In this paper, we present a set of cosmological constraints obtained from 5259 clusters of galaxies detected over an area of 12791 deg² in the western Galactic hemisphere of eROSITA’s first All-Sky Survey (eRASS1). The common footprint region (4968 deg²) between the eROSITA Survey and Dark Energy Survey (DES), the Kilo-Degree Survey (KiDS), and the Hyper Supreme Camera (HSC) survey is used for calibration of the scaling between X-ray count rate of the clusters and their total mass through measurements of their weak gravitational lensing signal. The eRASS1 cluster abundances constrain the ΛCDM parameters, namely, the energy density of the total matter to Ω_m = 0.29_−0.02^+0.01 and the normalization of the density fluctuations to σ₈ = 0.88 ± 0.02, and their combination yields S₈ = σ₈(Ω_m/0.3)^0.5 = 0.86 ± 0.01. These results are consistent and achieve at a similar precision with state-of-the-art cosmic microwave background (CMB) measurements. Furthermore, the eRASS1 cosmological experiment places a most stringent upper limit on the summed masses of left-handed light neutrinos to ∑ m_v < 0.43 eV (95% confidence interval) from cluster number counts alone. By combining eRASS1 cluster abundance measurements with CMB- and ground-based neutrino oscillation experiments, we measured the summed neutrino masses to be ∑ m_v = 0.09_−0.02^+0.04 eV or ∑ m_v = 0.12_−0.02^+0.03 eV, assuming a normal or inverted mass hierarchy scenario for neutrino eigenstates. The eRASS1 cluster abundances significantly improve the constraints on the dark energy equation of state parameter to w = −1.12 ± 0.12. When ∑ m_v and w are left free, we find consistent results with the concordance ΛCDM cosmology. Our results from the first All-Sky Survey improve the cosmological constraints by over a factor of 5 to 9 over the previous cluster surveys, establishing cluster abundance measurements for precision cosmology and setting the stage for deeper eROSITA All-Sky Surveys, as well as for future cluster abundance experiments.

Context. Relativistic electrons and magnetic fields permeate the intra-cluster medium (ICM) and manifest themselves as diffuse sources of synchrotron emission observable at radio wavelengths, namely radio halos and radio relics. Although there is broad consensus that the formation of these sources is connected to turbulence and shocks in the ICM, the details of the required particle acceleration, the strength and morphology of the magnetic field in the cluster volume, and the influence of other sources of high-energy particles are poorly known. Aims. Sufficiently large samples of radio halos and relics, which would allow us to examine the variation among the source population and pinpoint their commonalities and differences, are still missing. At present, due to the physical properties of the sources and the capabilities of existing facilities, large numbers of these sources are easiest to detect at low radio frequencies, where they shine brightly. Methods. We examined the low-frequency radio emission from all 309 clusters in the second catalog of Planck Sunyaev Zel’dovich detected sources that lie within the 5634 deg² covered by the Second Data Release of the LOFAR Two-meter Sky Survey (LoTSS-DR2). We produced LOFAR images at different resolutions, with and without discrete sources subtracted, and created overlays with optical and X-ray images before classifying the diffuse sources in the ICM, guided by a decision tree. Results. Overall, we found 83 clusters that host a radio halo and 26 that host one or more radio relics (including candidates). About half of them are new discoveries. The detection rate of clusters that host a radio halo and one or more relics in our sample is 30 ± 11% and 10 ± 6%, respectively. Extrapolating these numbers, we anticipate that once LoTSS covers the entire northern sky it will provide the detection of 251 ± 92 clusters that host a halo and 83 ± 50 clusters that host at least one relic from Planck clusters alone. All images and results produced in this work are publicly available via the project website.

The first SRG/eROSITA All-Sky Survey (eRASS1) provides the largest intracluster medium-selected galaxy cluster and group catalog covering the western Galactic hemisphere. Compared to samples selected purely on X-ray extent, the sample purity can be enhanced by identifying cluster candidates using optical and near-infrared data from the DESI Legacy Imaging Surveys. Using the red-sequence-based cluster findereROMaPPer, we measured individual photometric properties (redshiftz_λ, richnessλ, optical center, and BCG position) for 12000 eRASS1 clusters over a sky area of 13 116 deg², augmented by 247 cases identified by matching the candidates with known clusters from the literature. The median redshift of the identified eRASS1 sample isz= 0.31, with 10% of the clusters atz> 0.72. The photometric redshifts have an accuracy ofδz/(1 +z) ≲ 0.005 for 0.05 <z< 0.9. Spectroscopic cluster properties (redshiftz_specand velocity dispersionσ) were measured a posteriori for a subsample of 3210 and 1499 eRASS1 clusters, respectively, using an extensive compilation of spectroscopic redshifts of galaxies from the literature. We infer that the primary eRASS1 sample has a purity of 86% and optical completeness >95% forz> 0.05. For these and further quality assessments of the eRASS1 identified catalog, we applied our identification method to a collection of galaxy cluster catalogs in the literature, as well as blindly on the full Legacy Surveys covering 24069 deg². Using a combination of these cluster samples, we investigated the velocity dispersion-richness relation, finding that it scales with richness as log(λ_norm) = 2.401 × log(σ) − 5.074 with an intrinsic scatter ofδ_in= 0.10 ± 0.01 dex. The primary product of our work is the identified eRASS1 cluster catalog with high purity and a well-defined X-ray selection process, opening the path for precise cosmological analyses presented in companion papers.

Context.The circumgalactic medium (CGM) provides the material needed for galaxy formation and influences galaxy evolution. The hot (T > 10⁶K) CGM is poorly detected around galaxies with stellar masses (M_*) lower than 3 × 10¹¹ M_⊙due to the low surface brightness.Aims.We aim to detect the X-ray emission from the hot CGM around Milky Way-mass (MW-mass, log(M_*/M_⊙) = 10.5 − 11.0) and M31-mass (log(M_*/M_⊙) = 11.0 − 11.25) galaxies, in addition to measuring the X-ray surface brightness profile of the hot CGM.Methods.We applied a stacking technique to gain enough statistics to detect the hot CGM. We used the X-ray data from the first four SRG/eROSITA All-Sky Surveys (eRASS:4). We discussed how the satellite galaxies could bias the stacking and the method we used to carefully build the central galaxy samples. Based on the SDSS spectroscopic survey and halo-based group finder algorithm, we selected central galaxies with spectroscopic redshifts ofz_spec < 0.2 and stellar masses of 10.0 < log(M_*/M_⊙) < 11.5 (85 222 galaxies) – or halo masses of 11.5 < log(M_200m/M_⊙) < 14.0 (125,512 galaxies). By stacking the X-ray emission around galaxies, we obtained the mean X-ray surface brightness profiles. We masked the detected X-ray point sources and carefully modeled the X-ray emission from the unresolved active galactic nuclei (AGN) and X-ray binaries (XRB) to obtain the X-ray emission from the hot CGM.Results.We measured the X-ray surface brightness profiles for central galaxies of log(M_*/M_⊙) > 10.0 or log(M_200m/M_⊙) > 11.5. We detected the X-ray emission around MW-mass and more massive central galaxies extending up to the virial radius (R_vir). The signal-to-noise ratio (S/N) of the extended emission around MW-mass (M31-mass) galaxy is about 3.1σ(4.7σ) withinR_vir. We used aβmodel to describe the X-ray surface brightness profile of the hot CGM (S_X, CGM). We obtained a central surface brightness of log(S_X,0[erg s⁻¹ kpc⁻²]) = 36.7_−0.4^+1.4 (37.1_−0.4^+1.5) andβ = 0.43_−0.06^+0.10 (0.37_−0.02^+0.04) for MW-mass (M31-mass) galaxies. For galaxies with log(M_200m/M_⊙) > 12.5, the extended X-ray emission is detected with S/N > 2.8σand theS_X, CGMcan be described by aβmodel withβ ≈ 0.4 and log(S_X,0[erg s⁻¹ kpc⁻²]) > 37.2. We estimated the baryon budget of the hot CGM and obtained a value that is lower than the prediction of ΛCDM cosmology, indicating significant gas depletion in these halos. We extrapolated the hot CGM profile measured withinR_virto larger radii and found that within ≈3R_vir, the baryon budget is close to the ΛCDM cosmology prediction.Conclusions.We measured the extended X-ray emission from representative populations of central galaxies around and above MW-mass out toR_vir. Our results set a firm footing for the presence of the hot CGM around such galaxies. These measurements constitute a new benchmark for galaxy evolution models and possible implementations of feedback processes therein.

Aims. Understanding how the properties of galaxies relate to the properties of the hot circum-galactic medium (CGM) around them can constrain galaxy evolution models. We aim to measure the scaling relations between the X-ray luminosity of the hot CGM and the fundamental properties (stellar mass and halo mass) of a galaxy. Methods. We measured the X-ray luminosity of the hot CGM based on the surface brightness profiles of central galaxy samples measured from Spectrum Roentgen Gamma (SRG)/eROSITA all-sky survey data. We related the X-ray luminosity to the galaxies’ stellar and halo mass, and we compared the observed relations to the self-similar model and intrinsic (i.e., not forward-modeled) output of the IllustrisTNG, EAGLE, and SIMBA simulations. Results. The average hot CGM X-ray luminosity (L_X, CGM) correlates with the galaxy’s stellar mass (M_*). It increases from (1.6 ± 2.1) × 10³⁹ erg s⁻¹ to (3.4 ± 0.3) × 10⁴¹ erg s⁻¹, when log(M_*) increases from 10.0 to 11.5. A power law describes the correlation as log(L_X, CGM) = (2.4 ± 0.1)×log(M_*)+(14.6 ± 1.5). The hot CGM X-ray luminosity as a function of halo mass is measured within log(M_500c) = 11.3 − 13.7, extending our knowledge of the scaling relation by more than two orders of magnitude. L_X, CGM increases with M_500c from (3.0 ± 1.6) × 10³⁹ erg s⁻¹ at log(M_500c) = 11.3 to (1.3 ± 0.1) × 10⁴² erg s⁻¹ at log(M_500c) = 13.7. The relation follows a power law of log(L_X, CGM) = (1.32 ± 0.05)×log(M_500c)+(24.1 ± 0.7). Our observations highlight the necessity of non-gravitational processes at the galaxy group scale while suggesting these processes are sub-dominant at the galaxy scale. We show that the outputs of current cosmological galaxy simulations generally align with the observational results uncovered here but with possibly important deviations in selected mass ranges. Conclusions. We explore, at the low mass end, the average scaling relations between the CGM X-ray luminosity and the galaxy’s stellar mass or halo mass, which constitutes a new benchmark for galaxy evolution models and feedback processes.