Counterpart identification and classification for eRASS1 and characterisation of the active galactic nuclei content

Context. Accurately accounting for the Active Galactic Nucleus (AGN) phase in galaxy evolution requires a large, clean AGN sample. This is now possible with SRG/eROSITA, which completed its first all-sky X-ray survey (eRASS1) on June 12, 2020. The public Data Release 1 (DR1, Jan 31, 2024) includes 930,203 sources from the western Galactic hemisphere. Aims. The data enable the selection of a large AGN sample and the discovery of rare sources. However, scientific return depends on accurate characterisation of the X-ray emitters, requiring high-quality multi-wavelength data. This paper presents the identification and classification of optical and infrared counterparts to eRASS1 sources. Methods. Counterparts to eRASS1 X-ray point sources were identified using Gaia DR3, CatWISE2020, and Legacy Survey DR10 (LS10) with the Bayesian NWAY algorithm and trained priors. Sources were classified as Galactic or extragalactic via a machine-learning model combining optical/IR and X-ray properties, trained on a reference sample. For extragalactic LS10 sources, photometric redshifts were computed using C IRCLEZ . Results. Within the LS10 footprint, all 656,614 eROSITA/DR1 sources have at least one possible optical counterpart; ∼570 000 are extragalactic and likely AGN. Half are new detections compared to AllWISE, Gaia, and Quaia AGN catalogues. Gaia and CatWISE2020 counterparts are less reliable, due to the survey’s shallowness and the limited amount of features available to assess the probability of being an X-ray emitter. In the Galactic plane, where the overdensity of stellar sources also increases the chance of associations, using conservative reliability cuts, we identified approximately 18 000 Gaia and 55 000 CatWISE2020 extragalactic sources. Conclusions. We have released three high-quality counterpart catalogues – plus the training and validation sets – as a benchmark for the field. These datasets have many applications, but in particular, they empower researchers to build AGN samples tailored for completeness and purity, accelerating the hunt for the Universe’s most energetic engines.

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Journal
Astronomy and Astrophysics
Published
2025-10-28
DOI
https://doi.org/10.1051/0004-6361/202556142
Citations
1
Primary Topic
Galaxies: Formation, Evolution, Phenomena
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article
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1.15
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article

Counterpart identification and classification for eRASS1 and characterisation of the active galactic nuclei content

F. Di Mille, J. Brink, A. Georgakakis, Rongpu Zhou et al.
1 citations
Astronomy and Astrophysics
Galaxies: Formation, Evolution, Phenomena
1.15
article

Counterpart identification and classification for eRASS1 and characterisation of the active galactic nuclei content

F. Di Mille, J. Brink, A. Georgakakis, Rongpu Zhou, D. Schlegel, Z. Igo, M. Gromadzki, Teng Liu, Sophia G. H. Waddell, R. Shirley, D. Hernández-Lang, F. Valdés, Hartmut Winkler, J. Snigula, G. Lamer, B. A. Weaver, J. Wolf, M. Salvato, J. Robrade, M. Schramm, S. Freund, D. A. H. Buckley, F. Mannucci, M. Fabricius, T. Dwelly, J. Comparat, Dustin Lang, M. Krumpe, A. Rau, K. Nandra, I. Márquez, D. Homan, Johannes Büchner, A. Merloni, M. Brusa, M. Scialpi, A. Dey, A. Schwope, A. Zenteno, S. Ciroi, W. Roster, H. Starck, Shubhanshu Tiwari, Fabian Balzer, Adam D. Myers, M Kluge, S. Juneau
article en
1 citations

Abstract

Context. Accurately accounting for the Active Galactic Nucleus (AGN) phase in galaxy evolution requires a large, clean AGN sample. This is now possible with SRG/eROSITA, which completed its first all-sky X-ray survey (eRASS1) on June 12, 2020. The public Data Release 1 (DR1, Jan 31, 2024) includes 930,203 sources from the western Galactic hemisphere. Aims. The data enable the selection of a large AGN sample and the discovery of rare sources. However, scientific return depends on accurate characterisation of the X-ray emitters, requiring high-quality multi-wavelength data. This paper presents the identification and classification of optical and infrared counterparts to eRASS1 sources. Methods. Counterparts to eRASS1 X-ray point sources were identified using Gaia DR3, CatWISE2020, and Legacy Survey DR10 (LS10) with the Bayesian NWAY algorithm and trained priors. Sources were classified as Galactic or extragalactic via a machine-learning model combining optical/IR and X-ray properties, trained on a reference sample. For extragalactic LS10 sources, photometric redshifts were computed using C IRCLEZ . Results. Within the LS10 footprint, all 656,614 eROSITA/DR1 sources have at least one possible optical counterpart; ∼570 000 are extragalactic and likely AGN. Half are new detections compared to AllWISE, Gaia, and Quaia AGN catalogues. Gaia and CatWISE2020 counterparts are less reliable, due to the survey’s shallowness and the limited amount of features available to assess the probability of being an X-ray emitter. In the Galactic plane, where the overdensity of stellar sources also increases the chance of associations, using conservative reliability cuts, we identified approximately 18 000 Gaia and 55 000 CatWISE2020 extragalactic sources. Conclusions. We have released three high-quality counterpart catalogues – plus the training and validation sets – as a benchmark for the field. These datasets have many applications, but in particular, they empower researchers to build AGN samples tailored for completeness and purity, accelerating the hunt for the Universe’s most energetic engines.

Astronomy and AstrophysicsVol. 704
University of Wyoming (US), Lawrence Berkeley National Laboratory (US), University of Cape Town (ZA), Universität Hamburg (DE), Perimeter Institute (CA), University of Trento (IT), University of Johannesburg (ZA), University of Cambridge (GB), University of the Free State (ZA), Leibniz Institute for Astrophysics Potsdam (DE), Arcetri Astrophysical Observatory (IT), Istituto Nazionale di Fisica Nucleare, Galileo Galilei Institute for Theoretical Physics (IT), Max Planck Institute for Extraterrestrial Physics (DE), Excellence Cluster Origins (DE), Max Planck Institute for Astronomy (DE), South African Radio Astronomy Observatory (ZA), National Observatory of Athens (GR), Institute of Astronomy (RU), Cerro Tololo Inter-American Observatory (CL), NSF NOIRLab (US), Las Campanas Observatory (CL), Astronomical Observatory (RS), Instituto de Astrofísica de Andalucía (ES), Osservatorio di Astrofisica e Scienza dello Spazio, University of Florence (IT), University of Warsaw (PL), Ludwig-Maximilians-Universität München (DE), University of Bologna (IT)
Openalex Percentile: Top 15%
Galaxies: Formation, Evolution, Phenomena
1.15
DOIOpen Access PDFOpenAlex
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