README

Status of the X-CLASS Database

(last update 14th April 2026)

The construction of the X-CLASS catalogue from the XMM-Newton Science Archive and the C1 cluster selection are described in detail by Clerc et al. (2012b) and Koulouridis et al. (2021). A major update of the optical identification is described in Moysan et al. (2026). We summarize below, the content of the current online catalogue.

• 4176 XMM-Newton archival observations until August 2015 (window mode observations were not processed as well as observations for which one focal plane is missing)
• All observations are clipped to exposure times of 10 and 20 ks to obtain uniformity and span∼269 deg2 across the high-Galactic latitude (|b|>20°) sky.
• 1646 cluster candidates are retained after data screening.
• In Appendix D of Koulouridis et al. (2021), we listed 87 sources as "provisional". These were reinspected with deeper photometric data by Moysan et al. (2026). 40 of them made it to the final catalogue, the other 47 were removed.
• 1599 well-selected clusters are available online for the public.
• A set of three-digit flags indicate the reliability of the optical association and redshift.
• For each cluster, the information on X-ray detection as well as the X-ray and optical images overlaid by X-ray contours are available.
• The catalogue provides the cluster count-rates measured within an aperture in full exposure and in six energy bands [0.5 - 2], [2 - 10], [0.5 - 0.9], [1.3 - 2], [2 - 5] and [5 - 7] keV.
• The sample spans a wide redshift range, from the local Universe up to z∼1.5. 1060 spectroscopically confirmed clusters, 460 with photometric redshift only.
• Redshifts were obtained from the public astronomical resources (NED, CDS, SDSS skyserver etc.) and from the literature and dedicated follow-up, especially by:
  • Mehrtens et al 2012, pertaining to the XCS catalogue (comparison between the X-CLASS and XCS catalogues can be found in Clerc et al., 2012b)
  • Sadibekova et al., 2014, the X-CLASS - redMaPPer sample
  • Clerc et al., 2016, pertaining to the SPIDERS survey
  • Ridl et al., 2017, the GROND observations of the X-CLASS clusters
  • Moysan et al., 2026, the Legacy Survey DR10 and the MISTRAL spectroscopic follow-up
• A dedicated NED page displays a sky field (through Aladin Sky Atlas tool) with the coincidences within a radius of 3 arcmin around the X-ray centroid. Only extragalactic objects with a known redshift are listed.

The first X-CLASS catalogue & Cosmology

• The first catalogue release (Clerc et al., 2012b) comprised 845 clusters.
• 347 clusters were used for the cosmological analysis with the following selection criteria:
• 0 < off-axis <10’
• 40 < Ext_LH < 5000
• 0.009 < CR < 0.5

Notes

Related publications

• Moysan et al., 2026, A&A in press: Identification of low redshift groups and clusters of galaxies in the X-CLASS survey and the X-ray luminosity-temperature relation
• Koulouridis et al., 2021, A&A 652, A12: The X-CLASS survey: A catalogue of 1646 X-ray-selected galaxy clusters up to z<1.5
• The full list of X-CLASS related publications is accessible from this page.

Please carefully read the user guides below before using our cluster catalogue.
• the general documentation on the database,
• the description of the catalogue columns.
If you detect inexact information or for any questions concerning database content, please contact the database manager.








Acknowledgments

This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.
We gratefully acknowledge support from the CNRS/IN2P3 Computing Center (Lyon - France) for providing computing and data-processing resources needed for this work.
This research has made use of "Aladin sky atlas" developed at CDS, Strasbourg Observatory, France.
This work was supported by the ‘‘Programme National Cosmology et Galaxies'' (PNCG) of CNRS/INSU with INP and IN2P3, co-funded by CEA and CNES.
The Legacy Surveys consist of three individual and complementary projects: the Dark Energy Camera Legacy Survey (DECaLS; Proposal ID #2014B-0404; PIs: David Schlegel and Arjun Dey), the Beijing-Arizona Sky Survey (BASS; NOAO Prop. ID #2015A-0801; PIs: Zhou Xu and Xiaohui Fan), and the Mayall z-band Legacy Survey (MzLS; Prop. ID #2016A-0453; PI: Arjun Dey). DECaLS, BASS and MzLS together include data obtained, respectively, at the Blanco telescope, Cerro Tololo Inter-American Observatory, NSF’s NOIRLab; the Bok telescope, Steward Observatory, University of Arizona; and the Mayall telescope, Kitt Peak National Observatory, NOIRLab. Pipeline processing and analyses of the data were supported by NOIRLab and the Lawrence Berkeley National Laboratory (LBNL). The Legacy Surveys project is honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.
NOIRLab is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. LBNL is managed by the Regents of the University of California under contract to the U.S. Department of Energy.
This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho de Amparo, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and the Ministerio da Ciencia, Tecnologia e Inovacao, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy Survey. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenossische Technische Hochschule (ETH) Zurich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciencies de l’Espai (IEEC/CSIC), the Institut de Fisica d’Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig Maximilians Universitat Munchen and the associated Excellence Cluster Universe, the University of Michigan, NSF’s NOIRLab, the University of Nottingham, the Ohio State University, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University.
BASS is a key project of the Telescope Access Program (TAP), which has been funded by the National Astronomical Observatories of China, the Chinese Academy of Sciences (the Strategic Priority Research Program “The Emergence of Cosmological Structures” Grant # XDB09000000), and the Special Fund for Astronomy from the Ministry of Finance. The BASS is also supported by the External Cooperation Program of Chinese Academy of Sciences (Grant # 114A11KYSB20160057), and Chinese National Natural Science Foundation (Grant # 12120101003, # 11433005).
The Legacy Survey team makes use of data products from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which is a project of the Jet Propulsion Laboratory/California Institute of Technology. NEOWISE is funded by the National Aeronautics and Space Administration.
The Legacy Surveys imaging of the DESI footprint is supported by the Director, Office of Science, Office of High Energy Physics of the U.S. Department of Energy under Contract No. DE-AC02-05CH1123, by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility under the same contract; and by the U.S. National Science Foundation, Division of Astronomical Sciences under Contract No. AST-0950945 to NOAO.
The Photometric Redshifts for the Legacy Surveys (PRLS) catalog used in this paper was produced thanks to funding from the U.S. Department of Energy Office of Science, Office of High Energy Physics via grant DE-SC0007914.