Top: distribution of all sources in the X-ray detection likelihood vs. X-ray extension (in arcsec) plane. All counterparts are shown in grey (label: CTPS to ROSAT/2RXS sources). Sources classified as “GALAXY” are coloured according to their p_any parameter (see Sect. 3 of Salvato et al. 2018, for a definition of p_any). The majority of the galaxies are either associated to a very low significance X-ray detection and thus just galaxies in the field (bottom part) or to extended sources (upper right part), indicating that they could be passive galaxies members of clusters or local (low redshift) extended galaxies. Bottom: central object in the figure is the counterpart associated to a 2RXS source, with a low p_any but high extension in the X-ray images. In fact, the 2RXS source in this case was extended and the associated galaxy is the central galaxy of a cluster at redshift 0.145. These type of sources populate the top/right quadrant in the top panel of the figure.