CN N = 1 → 0 spectra (black) and Gaussian fits (green) from NGC 253 on a local standard of rest (LSR) V_LSR = 0 km s^-1 frequency scale. Both spectra are smoothed to a channel spacing of ~8.5 km s^-1 (3.125 MHz). Lower panel: the J = 1/2 → 1 / 2 (left) and J = 3/2 → 1 /2 (right) groups of CN lines. Upper panel: the strongest feature is the 0₀ → 1_-1 E line of CH₃OH (methanol). Far left: the F₁ = 0, F₂ = 1 → 0 and F₁ = 1, F₂ = 1 → 1 groups of ¹³CN N = 1 → 0 lines. In between this spectral feature and the methanol line: the F₁ = 1, F₂ = 2 → 1 group of ¹³CN transitions (for CN and ¹³CN rest frequencies, see Skatrud et al. 1983; and Bogey et al. 1984). The emission on the righthand side of the methanol line near 108.9 GHz might be caused by SiS 6 → 5. Numbers at the foot of each spectral CN or ¹³CN feature provide expected relative intensities with respect to the weaker group of lines in case of optically thin emission under conditions of local thermodynamical equilibrium. For less sensitive CN spectra obtained with smaller bandwidths, see Fig. 1 of Henkel et al. (1993).