Redshifts of the continuum sources in the J0217−0820 field. Left: continuum image at 870 μm. Right: confirmed spectral lines associated with those continuum sources in ALMA band 3 and band 4. A zoom version with the additional lines is available in Fig. B.1. We have confirmed five redshift groups. The central blazar lies at z = 0.6, while the surrounding dusty starbursts are located at four different redshifts.