Projects and Collaborations
HIFLUGCS and eeHIFLUGCS
The HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) is an X-ray selected sample of 64 galaxy clusters from the ROSAT All-Sky Survey with a flux limit of 2 × 10 -11 erg s-1cm-2 in the (0.1-2.4) keV band and Galactic latitude |b| ≤ 20°. HIFLUGCS is the largest complete sample with full Chandra and XMM-Newton follow-up.
X-ray data are available for these clusters:
- From Chandra for all 64 clusters.
- From XMM-Newton for all 64 clusters.
- From Suzaku for more than 40% of the HIFLUGCS clusters.
- From ROSAT PSPC pointing observations for 80%.
Since these systems are the X-ray brightest clusters in the sky, they are also well-studied in other wavebands, e.g., in the radio and optical regime.
In the near future, the sample size of HIFLUGCS will be increased 6-fold, resulting in the new complete high quality sample: eeHIFLUGCS. This significantly increased statistics will enable substantial improvements in precision for several studies as well as qualitatively new tests.
Zhang, Y.-Y.; Andernach, H.; Caretta, C. A.; et al., A&A, 2011, 526, A105
Mittal, R.; Hicks, A.; Reiprich, T. H. & Jaritz, V., A&A, 2011, 532, A133
Laganá, T. F.; Zhang, Y.-Y.; Reiprich, T. H. & Schneider, P., ApJ, 2011, 743, 13
Zhang, Y.-Y.; Laganá, T. F.; Pierini, D.; Puchwein, E.; Schneider, P. & Reiprich, T. H., A&A, 2011, 535, A78
Hudson, D. S.; Mittal, R.; Reiprich, T. H.; et al., A&A, 2010, 513, A37
Mittal, R.; Hudson, D. S.; Reiprich, T. H.; et al., A&A, 2009, 501, 835-850
Chen, Y.; Reiprich, T. H.; Böhringer, H.; et al., A&A, 2007, 466, 805-812
Reiprich, T. H., A&A, 2006, 453, L39-L42
Ikebe, Y.; Reiprich, T. H.; Böhringer, H.; Tanaka, Y. & Kitayama, T., A&A, 2002, 383, 773-790
Reiprich, T. H. & Böhringer, H., ApJ, 2002, 567, 716-740
ADS listing of publications containing the word "HIFLUGCS"
eROSITA
The primary science driver for eROSITA is to study dark energy with about 100,000 galaxy clusters. eROSITA was launched in 2019 and completed four full-sky surveys at X-ray wavelengths. Galaxy clusters can be studied in their far outskirts using the all-sky survey data.
eROSITA consists of seven identical mirror modules, each with its own CCD camera, designed to perform the most comprehensive X-ray map of the sky to date. It is orbiting around the second Lagrange point and detected already in the first survey more than 900,000 X-ray sources.
Press release about the successful launch: here
First data release in 2024: here
Veronica et al. 2025: The SRG/eROSITA All-Sky Survey: Large-scale view of the Centaurus cluster
McCall et al. 2024: The SRG/eROSITA All-Sky Survey: View of the Virgo Cluster
Veronica et al. 2024: The eROSITA view of the Abell 3391/95 field. Cluster outskirts and filaments
Whelan at al. 2022: X-ray studies of the Abell 3158 galaxy cluster with eROSITA
Iljenkarevic et al. 2022: eROSITA spectro-imaging analysis of the Abell 3408 galaxy cluster
XRISM
The X-Ray Imaging and Spectroscopy Mission (XRISM) is an X-ray astronomy satellite launched in September 2023, by the Japan Aerospace Exploration Agency (JAXA) in collaboration with NASA and ESA . XRISM is designed to study the universe's hottest and most energetic phenomena, such as galaxy clusters, black holes, and supernova remnants. It has two instruments on board: Resolve, a microcalorimeter spectrometer with 36 pixels, providing unprecedented high-resolution X-ray spectroscopy and Xtend, a soft X-ray imager with a 38′ × 38′ field of view.
With XRISM's excellent spectral resolution, it can investigate the distribution and movement of hot gas in galaxy clusters and the intergalactic medium.
Euclid
Euclid is a 1.2m optical telescope launched in July 2023, designed to investigate the nature of dark energy through high-precision cosmological measurements. A key component of the mission is the Euclid Wide Survey covering ~14000 deg2. It will allow the discovery and study of several hundred thousand galaxy clusters and groups up to redshift z~2, enabling us to trace the growth of cosmic structure over time. The resulting sample of galaxy clusters and groups will be the largest sample at z>1, offering a powerful dataset to constrain models of dark energy, including potential time evolution. Furthermore, Euclid's deep fields (totaling ~50 deg2) will serve as a reference for future deep surveys.
Euclid delivers both: Deep optical imaging for weak gravitational lensing and therefore accurate galaxy cluster masses, and near-infrared data for precise color measurements essential for identifying high-redshift clusters.
Our group is involved in the Euclid galaxy cluster working package, focusing on the multiwavelength exploitation of the data and the calibration of mass-observable relations.
XMM-Newton
Besides the analysis of individual galaxy groups and clusters with data from the XMM-Newton telescope, our group is involved in two large-scale XMM projects:
XXL
XXL is a survey using the XMM-Newton telescope to cover two regions of 25 deg2 each, with 10 ks observations. This gives a point source sensitivity of 5 x 10-15 erg s-1 cm-2 in the (0.5 - 2) keV band. With more than 6 Ms it is the largest XMM-Newton project approved to date. 600 new galaxy clusters with redshifts up to 2 will be used to constrain the dark energy equation of state.
FornaX
Between August 2024 and Februay 2027, XMM-Newton will do a deep follow-up of the Euclid Deep Field Fornax (EDFF), covering 10 deg2 with an effective exposure time of 40 ks. This will yield a detection limit of 1 x 10-15 erg s-1 cm-2 in the (0.5 - 2) keV band. The follow-up will identify the X-ray counterparts in the Euclid field and study galaxy groups at intermediate redshifts.
ASKAP/EMU
The Evolutionary Map of the Universe (EMU, e.g., Norris et al. 2011, Hopkins et al. 2025) is a radio continuum survey performed with the Australian Square Kilometer Array Pathfinder (ASKAP). Because of its very high sensitivity (40 times NVSS) it will discover star forming galaxies up to redshift 1. Moreover, EMU will detect radio halos, radio relics, tailed radio galaxies, etc. in thousands of galaxy clusters. It's sister program POSSUM aims to measure the all-sky Faraday Rotation measures in order to study magnetic fields in various environments across the Universe.
NewAthena
The European Space Agency (ESA) has selected the Athena X-ray observatory for their next large (billion Euro-class) space mission.
After ESA decided to address the "Hot and Energetic Universe" Science Theme with their next major mission, the Advanced Telescope for High-Energy Astrophysics (Athena) space mission proposal has now been selected by ESA. One of the key science drivers of the Hot and Energetic Universe Science Theme is the study of the largest objects in the Universe, galaxy clusters, from ancient times, about ten billion years ago, when they first formed, down to the present. Most of the normal matter in these systems, intergalactic gas between the galaxies, is so hot, more than ten million degrees, that it is invisible in the optical but radiates at X-ray wavelengths. A team of astrophysicists from many institutes world-wide, including the Argelander-Institute for Astronomy at Bonn University, demonstrated that the X-ray mission Athena has the technical capabilities to solve the outstanding questions about galaxy cluster evolution.
In 2023, the ESA Science Programme Committee approved a revised version of the Athena X-ray observatory, now called NewAthena. Industrial work resumed in the second quarter of 2024, with the mission targeting formal adoption in 2027 and a planned launch in 2037. NewAthena will be the ultimate X-ray instrument to analyze all the details of the hot and energetic universe. The main science topics are galaxy clusters, the evolution of their physical properties, and the accretion process of black holes.
Further information: