Left: star formation rate – stellar mass plane between 5 ≤ z ≤ 6 showing the position of the main sequence of star-forming galaxies (black line; Speagle et al. 2014) and the z ∼ 5 galaxies with [C II] detections in the ALPINE survey (gray circles; Le Fèvre et al. 2020; Faisst et al. 2020). HZ4, shown as an orange diamond, lies only ∼0.2 dex above the main sequence and can be considered a typical star-forming galaxy at this redshift (Faisst et al. 2020). Right: continuum-subtracted [C II] 158 μm spectrum of HZ4 extracted inside a circular aperture of 0.9″ radius (∼5.4 kpc). The best Gaussian fit is centered at an observed frequency of ν₀ = 290.471 GHz, which corresponds to a redshift of z = 5.54.