Dust temperature T_d versus redshift for galaxies in the literature and this work. Left panel: dust temperatures in optically thin models. Right panel: dust temperatures in optically thick models for galaxies in Jin et al. (2019) and this work, and in optically thin T_d for remaining data points. This figure shows a population of cold dusty galaxies at z = 4–6 in optically thin models, but they might be comparably warm, as in main sequence galaxies, if they have optically thick dust.