Integrated spectrum of GN20 centered on Paα. Plotted in green is the standard deviation derived from a background aperture of the same size as the galaxy spectrum. The spectrum plotted in red shows the CO(2−1) emission extracted from the data presented in Hodge et al. (2012); overplotted in blue is the single Gaussian fit to derive the redshift (see text). The systemic velocity corresponds to a redshift of z = 4.0548 (Carilli et al. 2011).