Average mass distribution of gas densities inside clusters. The distribution is the mean of the mass-weighted histograms of gas densities inside clusters. The inside of a cluster is the spherical region enclosing all cluster members. The shaded region indicates the standard deviation around the mean value. The dashed vertical line indicates the density threshold we use for star formation. The top horizontal axis shows the ratio between the free-fall time and ϵ_ff = 0.1 (see Sect. 2.2 for more details).