2022MNRAS.511.1227G

The XXL Survey is the largest homogeneous survey carried out with XMM-Newton. Covering an area of 50 deg², the survey contains several hundred galaxy clusters out to a redshift of ≃2, above an X-ray flux limit of ∼6 x 10^–15 er g cm^–2 s^–1. The GAMA spectroscopic survey of ∼300 000 galaxies covers ≃286 deg², down to an r-band magnitude of r < 19.8 mag. The region of overlap of these two surveys (covering 14.6 deg²) represents an ideal opportunity to study clusters selected via two independent selection criteria. Generating two independently selected samples of clusters, one drawn from XXL (spanning a redshift range 0.05 <= z <= 0.3) and another from GAMA (0.05 <= z <= 0.2), both spanning 0.2 <= M₅₀₀ <= 5 x 10¹⁴ M_☉, we investigate the relationship between X-ray luminosity and velocity dispersion (L_X - σ_v relation). Comparing the L_X - σ_v relation between the X-ray selected and optically selected samples, when not accounting for the X-ray selection, we find that the scatter of the X-ray selected sample is 2.7 times higher than the optically selected sample (at the 3.7σ level). Accounting for the X-ray selection to model the L_X - σ_v relation, we find that the difference in the scatter increases (with the X-ray selected sample having a scatter 3.4 times larger than the optically selected sample). Although the scatter of the optically selected sample is lower, we find 13 optically selected GAMA groups undetected in X-rays. Inspection of the difference in magnitude between the first and second brightest galaxies in the cluster, and a stacked X-ray image of these 13 groups, suggests that these are young systems still in the process of forming.