The flux density (S = I/FWHM) of the CO and [C i] emitters versus redshift. The black stars show the current redshifted CO emitters which also exhibit [C i] emission (designated by coloured stars). The small squares show all of the other published z ≥ 1 CO emitters. The telescope labels show the expected rms noise level per each 50 km  s^-1 channel after 10 h of observation with the corresponding telescope at 100 GHz (see main text). The look-back time is calculated using H₀ = 71 km s^-1 Mpc^-1, Ω_matter = 0.27 and Ω_Λ = 0.73 (Spergel et al. 2003), which we use throughout the paper.