Most stars in today’s Universe reside within spheroids, which are bulges of spiral galaxies and elliptical galaxies^(1,2). Their formation is still an unsolved problem^(3-5). Infrared/submillimetre-bright galaxies at high redshifts⁶ have long been suspected to be related to spheroid formation^(7-12). Proving this connection has been hampered so far by heavy dust obscuration when focusing on their stellar emission^(13-15) or by methodologies and limited signal-to-noise ratios when looking …