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

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, andWAVES, 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? 109:8 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 halomass 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-theart 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.