A new View of Young galaxies with ALMA and JWST

The majority of the Universes star formation occurred shrouded in cosmic dust, making it extremely challenginghistoricallyto obtain a clear view of young (z2) galaxy assembly. While the Atacama Large Millimeter Array (ALMA) has already enabled significant progress including pioneering work by the PI to directly image this dusty star formation in early galaxies their existing stellar populations remain obscured. At the same time, our understanding of the molecular gas fueling that star formation is still limited to unresolved detections and/or biased tracers. Fortunately, we are on the verge of a breakthrough with the newly launched James Webb Space Telescope (JWST) and the imminent arrival of ALMA Bands 1&2. To this end, the PI has obtained a suite of competitive proprietary data on JWST, ALMA, and Band 1/2 pilot facilities. Fully capitalizing on this opportunity now requires a team of capable researchers. By leveraging this data and a novel resolved spectral energy distribution (SED) fitting technique, the proposed program will 1) reveal the hidden stellar populations in the dustiest young (z~2.5) galaxies for the first time, testing predictions from hierarchical galaxy formation; 2) detect and map the crucial CO(1-0) and CO(2-1) emission lines in a statistical sample of these same young galaxies, directly measuring their full cold gas reservoirs via state-of-the-art Bayesian modeling, and calibrating other less-well-understood gas tracers; and 3) push studies of dust and gas in galaxies to the earliest cosmic epochs (z>6.5), including detecting their molecular gas via CO(3-2)one of the key science goals for ALMA Band 1thus revealing how the most massive early galaxies grew so quickly. Given her demonstrated track record leading a group and the wealth of data secured, the PI is optimally placed to take advantage of this unique confluence of revolutionary instruments to provide a completely new view into how young galaxies built up their stellar masses.