Contour plot (blue) of the radio flux density F_ν (in milliJansky) in the R − B_G plane following the expression of R given in Eq. 1 for a frequency of 100 GHz and a source redshift z = 0.02. The red line is the corresponding NOEMA sensitivity for an integration time equal to the source size light travel time R/c. Above this line, the required exposure time to detect a flux density F_ν is lower than R/c. It is larger below. The left y scale is in centimeter units while the right y scale being in R_g units for a supermassive black hole mass of 10⁷ M_⊙.