The eROSITA Final Equatorial Depth Survey (eFEDS)

SDSS spectroscopic observations of X-ray sources

¹ Max Planck Institute for Extraterrestrial Physics, Gießenbachstraße 1, 85748 Garching, Germany
² Excellence Cluster ORIGINS, Boltzmannstrasse 2, D-85748 Garching, Germany
³ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
⁵ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁶ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
⁸ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁹ Center for Cosmology and Particle Physics, Department of Physics, 726 Broadway, Room 1005, New York University, New York, NY 10003, USA
¹⁰ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
¹¹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹² Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
¹³ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁴ Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
¹⁵ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
¹⁶ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
¹⁷ Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA 02138, USA
¹⁸ Instituto de Astronomía, Universidad Nacional Autónoma de México Campus Ensenada, Km 107, Carret. Tijuana-Ensenada, 22860 Ensenada, Mexico
¹⁹ Department of Astronomy, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
²⁰ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 70-264, 04510 México D. F., Mexico
²¹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
²² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
²³ National Center for Supercomputing Applications, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
²⁴ Trottier Space Institute & Department of Physics, McGill University, 3600 rue University, Montreal, QC H3A 2T8, Canada
²⁵ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁶ Apache Point Observatory and New Mexico State University, P.O. Box 59, Sunspot, NM 88349-0059, USA
²⁷ Sternberg Astronomical Institute, Moscow State University, Moscow, Russia
²⁸ Department of Astronomy, New Mexico State University, Las Cruces, NM 88003, USA
²⁹ Department of Physics & Astronomy, Western Washington University, 516 High Street, Bellingham, WA 98225, USA
³⁰ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
³¹ Institute for Astronomy & Astrophysics, National Observatory of Athens, V. Paulou & I, Metaxa 11532, Greece
³² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³³ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
³⁴ Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON M5S 98H, Canada
³⁵ Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
³⁶ Department of Physics, 196 Auditorium Road, Unit 3046, University of Connecticut, Storrs, CT 06269, USA

^⋆ Corresponding author: caydar@mpe.mpg.de

Received: 4 March 2025
Accepted: 11 April 2025

Abstract

We present one of the largest uniform optical spectroscopic surveys of X-ray selected sources to date that were observed as a pilot study for the Black Hole Mapper (BHM) survey. The BHM program of the Sloan Digital Sky Survey (SDSS)-V is designed to provide optical spectra for hundreds of thousands of X-ray selected sources from the SRG/eROSITA all-sky survey. This significantly improves our ability to classify and characterise the physical properties of large statistical populations of X-ray emitting objects. Our sample consists of 13 079 sources in the eROSITA eFEDS performance verification field, 12 011 of which provide reliable redshifts from 0 ≲ z ≤ 5.8. The vast majority of these objects were detected as point-like sources (X-ray flux limit F_{0.5 − 2 keV} ≳ 6.5 × 10⁻¹⁵ erg/s/cm²) and were observed for about 20 years with fibre-fed SDSS spectrographs. After including all available redshift information for the eFEDS sources from the dedicated SDSS-V plate programme and archival data, we visually inspected the SDSS optical spectra to verify the reliability of these redshift measurements and the performance of the SDSS pipeline. The visual inspection allowed us to recover reliable redshifts (for 99% of the spectra with a signal-to-noise ratio of > 2) and to assign classes to the sources, and we confirm that the vast majority of our sample consists of active galactic nuclei (AGNs). Only ∼3% of the eFEDS/SDSS sources are Galactic objects. We analysed the completeness and purity of the spectroscopic redshift catalogue, in which the spectroscopic completeness increases from 48% (full sample) to 81% for a cleaner, brighter (r_AB < 21.38) sample that we defined by considering a high X-ray detection likelihood, a reliable counterpart association, and an optimal sky coverage. We also show the diversity of the optical spectra of the X-ray selected AGNs and provide spectral stacks with a high signal-to-noise ratio in various sub-samples with different redshift and optical broad-band colours. Our AGN sample contains optical spectra of (broad-line) quasars, narrow-line galaxies, and optically passive galaxies. It is considerably diverse in its colours and in its levels of nuclear obscuration.