During this reporting period, the project has made substantial progress across all three work packages, leveraging new observations from ALMA and JWST as well as methodological innovations.
Revealing hidden stellar populations: Using JWST imaging in combination with existing ALMA data, we directly detected the underlying stars in heavily dust-obscured galaxies for the first time (Hodge et al. 2025). This breakthrough overcomes a decades-old limitation in the study of galaxy evolution. In parallel, we developed and tested a novel resolved spectral energy distribution (SED) fitting technique (Li et al. 2024), which allows us to construct spatially resolved maps of stellar and dust emission. Ongoing work applies this methodology to the newly observed dusty galaxies, shedding new insight into the internal structure and formation of vigorously star-forming galaxies at z ~ 2–3.
Tracing the molecular gas: A major activity has been the analysis of molecular gas reservoirs using CO and [CI] emission. In Frias Castillo et al. (2025), we presented one of the first direct comparisons of these tracers in unlensed high-redshift dusty galaxies, demonstrating the potential of [CI] as an alternative gas mass indicator. In Boogaard et al. (2025), we applied the innovative TUNER radiative transfer framework to the well-studied galaxy GN20, enabling self-consistent modeling of its CO and dust emission. This represents an important methodological advance, replacing simple multi-component models with a physically motivated gas density distribution. Ongoing work extends the cold gas observations to a statistically significant sample of distant galaxies.
Pushing to the earliest epochs: At z > 7, we mapped [CII] emission on sub-kiloparsec scales in the galaxy REBELS-25, revealing the most distant dynamically cold disk galaxy known to-date (Rowland et al. 2024). This result demonstrates that ordered rotating structures were already in place only 700 million years after the Big Bang, challenging models of early galaxy assembly. Ongoing work is extending these studies with complementary CO observations and dedicated JWST imaging.
Together, these activities mark major steps toward the project’s overall objectives: to build the first complete, high-resolution picture of how young galaxies formed their stars, acquired their gas, and developed their structures across cosmic time.
