MAGAZ3NE: Confirming Dust Deficiency and Quiescent Nature of Ultramassive Galaxies at 3 < z < 4 with ALMA Observations

A major challenge in identifying massive quiescent galaxies at z > 3 is distinguishing truly passive systems from dust-obscured star-forming galaxies, as both populations exhibit similar red ultraviolet-to-near-infrared colors. In this work, we present Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 continuum observations of five ultramassive galaxies (UMGs; log ( M ⋆ / M ⊙ ) > 11 ) from the Massive Ancient Galaxies At z > 3 NEar-infrared survey, spectroscopically confirmed at z spec > 3. One UMG is weakly detected at 872 μ m while the remaining four UMGs are undetected down to the 3 σ depth. By incorporating ALMA constraints and far-infrared-to-radio photometry into the spectral energy distribution (SED) analysis, we demonstrate that all five UMGs are quiescent, lying significantly 1 dex below the star-forming main sequence. We estimate dust masses using both SED modeling and modified blackbody fitting, with consistent results between the two methods. Our results reveal the diversity in dust content among the quiescent UMGs at z > 3. While the lone ALMA-detected UMG retains a modest dust reservoir, the majority have dust-to-stellar mass ratios of M dust / M ⋆ ≲ 10 −4 , placing them among the most massive and extremely dust-poor spectroscopically confirmed quiescent galaxies known at z > 3. Our results support an evolutionary scenario of rapid quenching and dust removal for the most massive galaxies in the early Universe.