The dark side of early galaxies: geko uncovers dark-matter fractions at z ∼ 4 − 6

James Webb Space Telescope (JWST)/NIRCam slitless spectroscopy enables dynamical mass measurements for typical star-forming galaxies only a billion years after the big bang. We model the H morpho-kinematics of 163 galaxies at redshift –6 from FRESCO and CONGRESS (with JADES imaging), using the geko code, and infer rotational velocities and dispersions within . Our sample spans –10 and –11. Gas masses are inferred from empirical scaling relations and combined with stellar masses to yield baryonic masses. The resulting median inferred gas-to-baryonic mass fraction is . Using these baryonic masses together with the dynamical masses, we derive dark-matter fractions within the H half-light radius, and find a high median value of , where is defined relative to the total (DM + baryonic) mass. About two-thirds of systems are DM-dominated within –1 kpc. We find that decreases with stellar mass, consistent with predictions from simulations. The stellar Tully–Fisher relation shows a tentative offset to higher at fixed and substantial intrinsic scatter, suggesting that the relation is only beginning to emerge at . We measure a negative correlation between and baryonic surface density , weaker but broadly consistent with trends at cosmic noon and at . Qualitatively comparing with modified NFW profiles coupled to an empirical stellar-to-halo mass relation suggests that the lowest () require cored inner DM profiles, while the highest fractions favour cuspier profiles, potentially reflecting adiabatic contraction. Overall, the elevated and at are compatible with progenitors of baryon-dominated systems at and naturally anticipate overmassive black holes at fixed .