MNRAS 497, 3976–3992 (2020) doi:10.1093/mnras/staa2066 Advance Access publication 2020 July 15 SPIDERS: overview of the X-ray galaxy cluster follow-up and the final spectroscopic data release N. Clerc , 1‹ C. C. Kirkpatrick , 2,3 A. Finoguenov , 2,4 R. Capasso , 5 J. Comparat, 4 S. Damsted, 2 K. Furnell, 6 A. E. Kukkola, 2 J. Ider Chitham , 4 A. Merloni, 4 M. Salvato , 4 A. Gueguen, 4 T. Dwelly , 4 C. Collins, 6 A. Saro , 7,8,9 G. Erfanianfar, 4 D. P. Schneider, 10,11 J. Brownstein , 12 G. A. Mamon , 13 N. Padilla, 14 E. Jullo 15 and D. Bizyaev 16 1 IRAP, Universit´ e de Toulouse, CNRS, UPS, CNES, F-31028 Toulouse, France 2 Department of Physics, University of Helsinki, Gustaf H¨ allstr¨ omin katu 2, FI-00014 Helsinki, Finland 3 Helsinki Institute of Physics, Gustaf H ¨ allstr¨ omin katu 2, FI-00014 Helsinki, Finland 4 Max-Planck-Institut f¨ ur extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany 5 The Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, SE-106 91 Stockholm, Sweden 6 Astrophyics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK 7 Astronomy Unit, Department of Physics, University of Trieste, via Tiepolo 11, I-34131 Trieste, Italy 8 IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy 9 INAF – Osservatorio Astronomico di Trieste via G.B. Tiepolo 11, I-34143 Trieste, Italy 10 Department of Astronomy and Astrophysics, 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 and Astronomy, University of Utah, 115 S. 1400 E., Salt Lake City, UT 84112, USA 13 Institut d’Astrophysique de Paris (UMR 7095: CNRS & Sorbonne Universit´ e), F-75014 Paris, France 14 Instituto de Astrof´ ısica, Pontificia Universidad Cat´ olica de Chile, Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile 15 Aix Marseille Univ, CNRS, CNES, LAM, F-13388 Marseille, France 16 Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349, USA Accepted 2020 July 10. Received 2020 July 10; in original form 2020 April 16 ABSTRACT SPIDERS (The SPectroscopic IDentification of eROSITA Sources) is a large spectroscopic programme for X-ray selected galaxy clusters as part of the Sloan Digital Sky Survey-IV (SDSS-IV). We describe the final data set in the context of SDSS Data Release 16 (DR16): the survey overall characteristics, final targeting strategies, achieved completeness, and spectral quality, with special emphasis on its use as a galaxy cluster sample for cosmology applications. SPIDERS now consists of about 27 000 new optical spectra of galaxies selected within 4000 photometric red sequences, each associated with an X-ray source. The excellent spectrograph efficiency and a robust analysis pipeline yield a spectroscopic redshift measurement success rate exceeding 98 per cent, with a median velocity accuracy of 20 km s −1 (at z = 0.2). Using the catalogue of 2740 X-ray galaxy clusters confirmed with DR16 spectroscopy, we reveal the 3D map of the galaxy cluster distribution in the observable Universe up to z ∼ 0.6. We highlight the homogeneity of the member galaxy spectra among distinct regions of the galaxy cluster phase space. Aided by accurate spectroscopic redshifts and by a model of the sample selection effects, we compute the galaxy cluster X-ray luminosity function and we present its lack of evolution up to z = 0.6. Finally we discuss the prospects of forthcoming large multiplexed spectroscopic programmes dedicated to follow up the next generation of all-sky X-ray source catalogues. Key words: galaxies: clusters: general – cosmology: observations – X-rays: galaxies: clusters. 1 INTRODUCTION Studies of the distribution of galaxy clusters in the Universe rely on large and well-selected samples of astrophysical objects tracing the most massive nodes of the cosmic web. Large amounts of hot gas trapped within their deep matter potential wells make clusters of galaxies luminous X-ray sources (e.g. Jones & Forman 1999). They appear as diffuse extended sources (Abell 1958; Bahcall & Cen 1993;B¨ ohringer et al. 2004; Ridl et al. 2017; Oguri et al. 2018; ⋆ E-mail: nicolas.clerc@irap.omp.eu Ricci et al. 2018; Wen & Han 2018; Andreon et al. 2019; Gozaliasl et al. 2019), with galaxy overdensities extending out to over 10 Mpc (Trevisan, Mamon & Stalder 2017). Galaxies within haloes may have their properties affected by encounters with the hot gas (e.g. Popesso et al. 2015; Lotz et al. 2019; Owers et al. 2019). Clusters of galaxies are rare objects, the more massive, the rarer. Their number density scales between 10 −5 and 10 −8 Mpc −3 for lower mass limits of 10 14 and 10 15 M ⊙ , respectively (e.g. Vikhlinin et al. 2009). X-ray surveys of the extragalactic sky fulfil the requirements of a large volume coverage, high completeness, and high purity as required by cluster studies. Moreover, X-ray properties readily scale with the mass of the host systems (e.g. Kaiser 1986); observations C  2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnras/article/497/3/3976/5871833 by guest on 04 February 2022