a) Histogram of the surface density of all sources, the dark orange, orange, and yellow histograms correspond to the distance, mass, and both mass- and distance-limited subsamples, respectively. b) Histogram of L_bol/M [L_⊙/M_⊙]. The lines are the same as on panel a). c) Surface density versus ratio of M_vir/M. The colour codes are the same as for panels a) and b). Dotted lines show the line of M_vir = M, and a surface density of 1 g cm^-2. The properties of the selected subsamples are discussed in Sect. 3.5.