Extremely cold dusty galaxies at z=4-6: first direct evidence of CMB impact on high-z galaxy observables

We report observations of z=6-7 galaxy candidates IR-selected from FIR/submm observations, that are spectroscopically confirmed to be at z=3.62-5.85 by ALMA 3mm line scans. These galaxies have extremely cold dust temperatures, which mimicked much higher redshifts, with respect to known galaxies at z>4. Their dust temperatures are moderately above CMB temperatures, which thus plays a non-negligible role impacting their RJ beta-slopes and observed CO fluxes. High resolution ALMA imaging shows compact morphologies and evidences for mergers. This work reveals a population of cold dusty star-forming galaxies that were missing in current surveys. The physical mechanism accounting for their cold dust temperatures could be either rapid metal enrichment or low star formation efficiency