Left: A¹Π(ν′)  ← X¹Σ ⁺ (ν = 0) electronic bands of CO detected at z_abs = 1.774 towards the quasar SDSS J104705+205734. The modelled profile is over-plotted in red with the fitting residuals shown in grey. Short vertical lines indicate the branch (“R”, “P” or “Q”) and the rotational level of the transition (J = 0–3). Right: the corresponding excitation diagram of CO gives the column density of the rotational levels J weighted by their quantum degeneracy (g_J) versus the energy of these levels relative to the ground state. The excitation temperature is directly given by −1/(aln10) where a is the slope of the linear fit (solid blue line, with 1σ errors represented by the dotted curves).