Detecting clusters and groups of galaxies populating the local Universe in large optical spectroscopic surveys

¹ European Southern Observatory, Karl Schwarzschildstrasse 2, 85748 Garching bei München, Germany
² Excellence Cluster ORIGINS, Boltzmannstr. 2, D-85748 Garching bei München, Germany
³ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität, Scheinerstr.1, 81679 München, Germany
⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschildstr. 1, 85741 Garching bei München, Germany
⁵ Department of Physics & Astronomy, McMaster University, 1280 Main Street W, Hamilton, ON L8S 4M1, Canada
⁶ International Centre for Radio Astronomy Research, University of Western Australia, M468, 35 Stirling Highway, Perth, WA 6009, Australia
⁷ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
⁸ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere 61602, Estonia
⁹ Estonian Academy of Sciences, Kohtu 6, 10130 Tallinn, Estonia
¹⁰ Department of Astronomy, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
¹¹ Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 200240, China
¹² Key Laboratory for Particle Astrophysics and Cosmology (MOE) & Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai Jiao Tong University, Shanghai 200240, China
¹³ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
¹⁴ Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
¹⁵ Division of Science, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁶ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹⁷ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy
¹⁸ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
²⁰ Department of Physics, College of Natural and Mathematical Sciences, University of Dodoma, P.O. Box 338 Dodoma, Tanzania

^⋆ Corresponding author; ilaria.marini@eso.org

Received: 28 August 2024
Accepted: 22 December 2024

Abstract

Context. With the advent of wide-field cosmological surveys, samples of hundreds of thousands of spectroscopically confirmed galaxy groups and clusters are becoming available. While these large datasets offer a valuable tool to trace the baryonic matter distribution, controlling systematics in the identification of host dark-matter halos and estimating their properties remains crucial.

Aims. We intend to evaluate the predictions of retrieving the population of cluster and group of galaxies using three group-detection methods on a simulated dataset replicating the GAMA survey selection. Our goal is to understand the systematics and selection effects of each group finder, which will be instrumental for interpreting the unprecedented volume of spectroscopic data from SDSS, GAMA, DESI, and WAVES, and for leveraging optical catalogues in the (X-ray) eROSITA era to quantify the baryonic mass in galaxy groups.

Methods. We simulated a spectroscopic galaxy survey in the local Universe (down to z < 0.2 and stellar mass completeness M_⋆ ≥ 10^9.8 M_⊙) using a lightcone based on the cosmological hydrodynamical simulation Magneticum. We assessed the completeness and contamination levels of the reconstructed halo catalogues and analysed the reconstructed membership. Finally, we evaluated the halo-mass recovery rate of the group finders and explored potential improvements.

Results. All three group finders demonstrate high completeness levels (> 80%) on the galaxy group and cluster scales, confirming that optical selection is suitable for probing dense regions in the Universe. Contamination at the low-mass end (M₂₀₀ < 10¹³ M_⊙) is caused by interlopers and fragmentation. Galaxy membership is at least 70% accurate above the group-mass scale; however, inaccuracies can lead to systematic biases in halo-mass determination using the velocity dispersion of galaxy members. We recommend using other halo-mass proxies less affected by contamination – such as total stellar luminosity or mass – to recover accurate halo masses. Further analysis of the cumulative luminosity function of the galaxy members has shown remarkable accuracy in the group finders’ predictions of the galaxy population.

Conclusions. These results confirm the reliability and completeness of the spectroscopic catalogues compiled by these state-of-the-art group finders. This paves the way for studies that require large sets of spectroscopically confirmed galaxy groups and clusters or studies of galaxy evolution in different environments.