The Origin of Excessively Massive Galaxies at Cosmic Dawn

The first observations from the James Web Space Telescope (JWST) indicate a puzzling excess of super-luminous galaxies in the first half a Gigayear after the Big Bang. This is consistent with efficient conversion of the accreted gas into stars, unlike the lower efficiency at later times and smaller masses due to stellar and supernova feedback. While some may interpret this apparent discrepancy as a call for modifying the standard cosmological model, I will show that the high densities and low metallicities at this epoch actually guarantee a high star-formation efficiency in the most massive dark-matter haloes at this epoch. I will describe the conditions for feedback-free starbursts, demonstrate that they are expected to be valid for massive galaxies in the early Universe, and provide observable predictions for the continuing observations by JWST.