If Dark Matter is Fuzzy, the First Stars Form in Massive Pancakes

Fuzzy dark matter (FDM) is a proposed modification for the standard cold dark matter (CDM) model motivated by small-scale discrepancies in low-mass galaxies. Composed of ultralight (mass ∼ 1022 eV) axions with kiloparsec-scale de Broglie wavelengths, this is one of a class of candidates that predicts that the first collapsed objects form in relatively massive dark matter halos. This implies that the formation history of the first stars and galaxies would be very different, potentially placing strong constraints on such models. Here we numerically simulate the formation of the first stars in an FDM cosmology, following the collapse in a representative volume all the way down to primordial protostar formation including a primordial nonequilibrium chemical network and cooling for the first time. We find two novel results: first, the large-scale collapse results in a very thin and flat gas "pancake"; second, despite the very different cosmology, this pancake fragments until it forms protostellar objects indistinguishable from those in CDM. Combined, these results indicate that the first generation of stars in this model are also likely to be massive and, because of the sheet morphology, do not self-regulate, resulting in a massive Population III starburst. We estimate the total number of first stars forming in this extended structure to be 104 over 20 Myr using a simple model to account for the ionizing feedback from the stars, and should be observable with the James Webb Space Telescope. These predictions provide a potential smoking gun signature of FDM and similar dark matter candidates.