Introducing the PHOEBOS simulation: galaxy properties at the dawn of galaxy formation.

The James Webb Space Telescope (JWST) enables observations of galaxies at the end of cosmic dawn ($z \sim 10$–15) with unprecedented detail, revealing their properties. These observations offer crucial insights into the physical processes driving early galaxy formation. In this work, we introduce the Phoebos hydrodynamical cosmological simulation, a state-of-the-art 100 Mpc volume designed to study galaxy formation from cosmic dawn into the epoch of reionization. Phoebos includes a stellar feedback model that is intentionally weak, in order to address the high abundance of massive galaxies seen by JWST at early epochs. At variance with most large cosmological hydrodynamical simulations, we do not employ an effective equation of state model, instead our radiative cooling model allows us to capture the multiphase nature of the gas inside and around galaxies. Phoebos reproduces key observables of early galaxy formation at $z \gtrsim 8$, including the stellar mass function and the stellar-to-halo mass relation. It also recovers the observed slope of the stellar size-to-mass relation and matches the specific star formation rate remarkably well. These results suggest that efficient star formation in the presence of only mild regulation from stellar feedback drives early galaxy growth, supporting a scenario of rapid stellar mass assembly during cosmic dawn. However, signs in the cosmic star formation density suggest that, at lower redshifts, Phoebos might overpredict stellar masses within the systems, indicating a need for stronger feedback to reproduce later-time observations. This highlights the potential of Phoebos to interpret JWST observations and to probe the evolving physical processes that shape galaxy formation.