The XXL Survey. XLII. The L_X- σ_v relation of galaxy groups and clusters detected in the XXL and GAMA surveys

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 × 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 × 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.