Comparison between the gas mass derived from ATLASGAL fluxes and the virial mass. In the upper panel we show M vs. M_vir, in the lower one M vs. α_vir (=M_vir/M). Each group of sources is shown with a different symbol and colour, as indicated. The stars refer to sources of subsample S3, while their colour still identifies the group (red = IRB, green = RMS, black = D8, and blue = D24). A typical uncertainty (for subsamples S1 and S2) is shown in the top left or right corner. The dashed and dotted lines indicate M_vir = M and M_vir = 2M, respectively (see text).