The eROSITA Final Equatorial-Depth Survey (eFEDS)

Identification and characterization of the counterparts to point-like sources^★

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany
e-mail: mara@mpe.mpg.de
² Exzellenzcluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany
³ Institute for Astronomy and Astrophysics, National Observatory of Athens, V. Paulou and I. Metaxa, 11532 Greece
⁴ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁶ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, 606-8502 Sakyo-ku, Kyoto, Japan
⁷ Academia Sinica Institute of Astronomy and Astrophysics, 10617 Taipei, Taiwan
⁸ Research Center for Space and Cosmic Evolution, Ehime University, 2–5 Bunkyo-cho, Matsuyama, 790–8577 Ehime, Japan
⁹ Leibniz-Institut für Astrophysik Potsdam (AIP)., An der Sternwarte 16., 14482 Potsdam, Germany
¹⁰ Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹¹ Institute for Advanced Research, Nagoya University Furocho, Chikusa-ku, 464–8602 Nagoya, Japan
¹² Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr 1, 81679 Munich, Germany
¹³ Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
¹⁴ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
¹⁵ Department of Physics, University of Connecticut, 2152 Hillside Road, Unit 3046, Storrs, CT 06269, USA
¹⁶ Department of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
¹⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹⁸ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, 16802 University Park, PA, USA
¹⁹ Department of Physics and Yale Center for Astronomy and Astrophysics, Yale University, PO Box 208120, New Haven, CT 065208120, USA
²⁰ Instituto de Astronomia Ensenada, Universidad Nacional Autónoma de Mexico, AP 106, Ensenada 22800, Mexico
²¹ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980–8578, Japan
²² Astronomical Institute, Tohoku University, Aramaki, Aoba-ku, Sendai, Miyagi 980–8578, Japan
²³ Department of Physics and Astronomy, University of Utah, 115 S. 1400 E., Salt Lake City, UT 84112, USA
²⁴ Centro de Investigación en Astronomia, Universidad Bernardo O’Higgins, Avenida Viel 1497, Santiago, Chile
²⁵ Apache Point Observatory and New Mexico State University, PO Box 59, Sunspot, NM 88349, USA
²⁶ Centro de Astronomia (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta 1270300, Chile

Received: 25 June 2021
Accepted: 7 March 2022

Abstract

Context. In November 2019, eROSITA on board of the Spektrum-Roentgen-Gamma (SRG) observatory started to map the entire sky in X-rays. After the four-year survey program, it will reach a flux limit that is about 25 times deeper than ROSAT. During the SRG performance verification phase, eROSITA observed a contiguous 140 deg² area of the sky down to the final depth of the eROSITA all-sky survey (eROSITA Final Equatorial-Depth Survey; eFEDS), with the goal of obtaining a census of the X-ray emitting populations (stars, compact objects, galaxies, clusters of galaxies, and active galactic nuclei) that will be discovered over the entire sky.

Aims. This paper presents the identification of the counterparts to the point sources detected in eFEDS in the main and hard samples and their multi-wavelength properties, including redshift.

Methods. To identifyy the counterparts, we combined the results from two independent methods (NWAY and ASTROMATCH), trained on the multi-wavelength properties of a sample of 23k XMM-Newton sources detected in the DESI Legacy Imaging Survey DR8. Then spectroscopic redshifts and photometry from ancillary surveys were collated to compute photometric redshifts.

Results. Of the eFEDS sources, 24 774 of 27 369 have reliable counterparts (90.5%) in the main sample and 231 of 246 sourcess (93.9%) have counterparts in the hard sample, including 2514 (3) sources for which a second counterpart is equally likely. By means of reliable spectra, Gaia parallaxes, and/or multi-wavelength properties, we have classified the reliable counterparts in both samples into Galactic (2695) and extragalactic sources (22 079). For about 340 of the extragalactic sources, we cannot rule out the possibility that they are unresolved clusters or belong to clusters. Inspection of the distributions of the X-ray sources in various optical/IR colour-magnitude spaces reveal a rich variety of diverse classes of objects. The photometric redshifts are most reliable within the KiDS/VIKING area, where deep near-infrared data are also available.

Conclusions. This paper accompanies the eROSITA early data release of all the observations performed during the performance and verification phase. Together with the catalogues of primary and secondary counterparts to the main and hard samples of the eFEDS survey, this paper releases their multi-wavelength properties and redshifts.