Shedding Light on the Nature of Dark Matter and Dark Energy with Multi-Wavelength All-Sky Surveys

Project Information

DarkQuest

Grant agreement ID: 101002585

DOI

10.3030/101002585

EC signature date 18 December 2020

Start date 1 April 2021

End date 31 March 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 000 000,00

EU contribution

€ 2 000 000,00

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 3 - DarkQuest (Shedding Light on the Nature of Dark Matter and Dark Energy with Multi-Wavelength All-Sky Surveys)

Reporting period: 2024-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

Clusters of galaxies, formed from the highest density fluctuations in the early Universe, represent the largest virialized objects that have undergone gravitational collapse. Their abundance, as traced by the evolution of the halo mass function, reflects the growth of linear density perturbations and can therefore be utilized as a probe of cosmology. Cluster number counts do not display strong degeneracies among the primary late-time cosmological parameters while maintaining a comparable precision. The sum of the right-handed neutrino mass can also be constrained via cluster abundances, particularly in combination with external probes, as the cosmic neutrinos have a non-negligible effect on the growth of matter perturbations during the matter and dark energy-dominated era. The overall objective of the DarkQuest project is to constrain cosmological parameters and the dark sector using the eROSITA All-Sky Survey.

Launched on July 13, 2019, the eROSITA X-ray telescope, on board the Spectrum-Roentgen-Gamma (SRG) survey mission, is an X-ray telescope whose soft sensitivity is optimized for detecting galaxy clusters in the sky. The primary science goal of eROSITA is to constrain the cosmological parameters, describing our Universe with a percent-level precision. The primary objective of the ERC project DarkQuest is to constrain cosmological parameters and dark matter models using the eROSITA All-Sky Survey data in combination with the overlapping weak lensing surveys.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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).

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). 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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The eFEDS field served as a test survey to develop the necessary science software, including but not limited to the detection algorithm, catalog creation, X-ray processing, and the X-ray selection function. After the eFEDS publications, we focused on analyzing the first All-Sky Survey observations and developed the cosmology pipeline, which is applied to the survey data. These results will be the first precision cosmology measurements made by cluster counts. They will produce precision cosmological measurements, the energy density of total matter, and the normalization of density fluctuations at a 3-5% level. We expect to submit the cosmology paper on the first All-Sky Survey published next year.

Another major achievement beyond cosmology we expect to deliver before the end of the project is the detection of whim filament and large-scale structure with eROSITA. In this project, we stack eROSITA observations of more than 7000 optically selected filaments. This will be the first study to detect baryons from cosmic filaments, which will constrain the missing baryons in the Universe.

The locations of 12,247 clusters of galaxies detected in the first eROSITA All-Slky Survey.