Abstract JWST/NIRCam slitless spectroscopy enables dynamical mass measurements for typical star-forming galaxies only a billion years after the Big Bang. We model the Hα morpho-kinematics of 163 galaxies at redshift z ≈ 4-6 from FRESCO and CONGRESS (with JADES imaging), using the geko code, and infer rotational velocities and dispersions within re. Our sample spans log M⋆ ≈ 7-10 and log Mdyn ≈ 9-11. 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 〈fgas〉 = 0. 77. Using these baryonic masses together with the dynamical masses, we derive dark-matter fractions fDM (r re) within the Hα half-light radius, and find a high median value of 〈fDM〉 = 0. 73, where fDM is defined relative to the total (DM+baryonic) mass. About two-thirds of systems are DM-dominated within re ∼ 0. 5–1 kpc. We find that fDM decreases with stellar mass, consistent with predictions from simulations. The stellar Tully-Fisher relation shows a tentative offset to higher vcirc at fixed M⋆ and substantial intrinsic scatter, suggesting that the relation is only beginning to emerge at z ∼ 5. We measure a negative correlation between fDM and baryonic surface density Σbar, weaker but broadly consistent with trends at cosmic noon and at z ∼ 0. Qualitatively comparing with modified NFW profiles coupled to an empirical stellar-to-halo mass relation suggests that the lowest fDM (≲ 0. 4) require cored inner DM profiles, while the highest fractions favour cuspier profiles, potentially reflecting adiabatic contraction. Overall, the elevated fgas and fDM at z ≳ 4 are compatible with progenitors of baryon-dominated systems at z ∼ 2 and naturally anticipate overmassive black holes at fixed M⋆.
Danhaive et al. (2026) studied this question.