An Early Dark Matter–dominated Phase in the Assembly History of Milky Way–mass Galaxies Suggested by the TNG50 Simulation and JWST Observations

Abstract Whereas well-studied galaxies at cosmic noon are found to be baryon dominated within the effective radius, recent JWST observations of z ∼ 6–7 galaxies with stellar masses of only M * ∼ 10 8−9 M ⊙ surprisingly indicate that they are dark matter dominated within r e ≈ 1 kpc. Here, we place these high-redshift measurements in the context of the TNG50 galaxy formation simulation by measuring the central (within 1 kpc) stellar, gas, and dark matter masses of galaxies in the simulation. The central baryon fraction varies strongly with galaxy stellar mass in TNG50, and this M * dependence is remarkably constant across 0 < z < 6: galaxies of low stellar mass ( M * ∼ 10 8−9 M ⊙ ) are dark matter dominated as f baryon (<1 kpc) ∼ 0.25. At z = 6, the baryonic mass in the centers of low-mass galaxies is largely comprised of gas, exceeding the stellar mass component by a factor ∼4. We use the simulation to track the typical evolution of such low-mass, dark matter–dominated galaxies at z = 6 and show that these systems become baryon dominated in their centers at cosmic noon, with high stellar-to-gas mass ratios, and grow to galaxies of M * ∼ 10 10.5 M ⊙ at z = 0. Comparing to the dynamical and stellar mass measurements from observations at high redshifts, these findings suggest that the inferred star formation efficiency in the early Universe is broadly in line with the established assumptions for the cosmological simulations. Moreover, our results imply that the JWST observations may indeed have reached the early low-mass regime where the central parts of galaxies transition from being dark matter dominated to being baryon dominated.