R_3.6 ratio evolutionary tracks; in this plot each track is truncated at the redshift at which the radio flux density drops below the detection limit S_{1.4 GHz} ~ 100 μJy. The horizontal dashed lines indicate R_3.6 ratios of 100, 200, and 500. The errors associated to the measured R_3.6 ratios of the Collier et al. (2014) and Herzog et al. (2014) samples are of the same magnitude of the symbols reported in the plot, and not shown. At the end of the Arp 220 reddened track we indicate the star formation rate expected for this source if it were at redshift ~3, under the hypothesis that star-forming activity entirely accounts for the radio emission in this object.