2026ApJ..1000..159M

We investigate the dust mass buildup and star formation efficiency of two galaxies at z > 12-GHZ2 and GS-z14-0-by combining Atacama Large Millimeter/submillimeter Array and JWST observations with an analytical model that assumes dust at thermal equilibrium. We obtained 3σ constraints on the dust mass of $\mathrm{log}{M}_{{\rm{dust}}}/{M}_{\odot }\lt 5.0$ and <5.3, respectively. These constraints are in tension with a high dust condensation efficiency in stellar ejecta but are consistent with models with a short metal accretion timescale at z > 12. Given the young stellar ages of these galaxies (t_age ∼ 10 Myr), dust grain growth via accretion may still be ineffective at this stage, though it likely works efficiently to produce significant dust in galaxies at z ∼ 7. The star formation efficiencies, defined as the star formation rate divided by the molecular gas mass, reach ∼10 Gyr^–1 over a 10 Myr timescale, aligning with the expected redshift evolution of “starburst” galaxies with efficiencies that are ∼0.5-1 dex higher than those in main-sequence galaxies. This starburst phase seems to be common in UV-bright galaxies at z > 12 and is likely associated with the unique conditions of the early phases of galaxy formation, such as bursty star formation. Direct observations of molecular gas tracers like [C II] will be crucial to further understanding the nature of bright galaxies at z > 12.