Need for PRIMA to understand the nature and ISM physical conditions of HST-dark galaxies

Abstract: One of the main open issues in galaxy formation and evolution is the early assembly of the most massive galaxies and their contribution to the stellar mass and star formation rate densities at early epochs. Massive red sources already in place at z > 2 to 3 have been found in deep Spitzer-IRAC and ALMA surveys. They are often called optically and near-IR dark, or HST-dark, being undetected even in the deepest HST frames. The submillimeter (i.e., ALMA) detection of these sources confirms their high-z dusty nature: they are massive (e.g., log(M*/Msun) > 10) and dusty star-forming galaxies with estimated redshifts in the 2.5 to 7 range. They seem to lie mostly below the main sequence (MS) of star-forming galaxies and show gas depletion times <1 Gyr. Imaging with the PRIMA/PRIMAger instrument over the full 25 to 265 micron range will allow us to characterize their still uncovered spectral energy distributions between JWST and ALMA spectral windows, probing their dust content and properties (e.g., temperature, mass), whereas spectroscopic observations with FIRESS will be the key to investigate the nature of their powering source (e.g., AGN or star formation) and to study the physics of their ISM, by detecting and measuring fine structure lines in the mid- and far-IR domain.