The dark side of early galaxies: <scp>geko</scp> uncovers dark-matter fractions at <i>z</i> ∼ 4 − 6

Abstract 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$\alpha$ morpho-kinematics of 163 galaxies at redshift $z\approx 4$–6 from FRESCO and CONGRESS (with JADES imaging), using the geko code, and infer rotational velocities and dispersions within $r_{\rm e}$. Our sample spans $\log M_{\star }\approx 7$–10 and $\log M_{\rm dyn}\approx 9$–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 $\langle f_{\rm gas}\rangle = 0.77$. Using these baryonic masses together with the dynamical masses, we derive dark-matter fractions $f_{\rm DM}(r \lt r_{\rm e})$ within the H$\alpha$ half-light radius, and find a high median value of $\langle f_{\rm DM}\rangle = 0.73$, where $f_{\rm DM}$ is defined relative to the total (DM + baryonic) mass. About two-thirds of systems are DM-dominated within $r_{\rm e}\sim 0.5$–1 kpc. We find that $f_{\rm DM}$ decreases with stellar mass, consistent with predictions from simulations. The stellar Tully–Fisher relation shows a tentative offset to higher $v_{\rm circ}$ at fixed $M_{\star }$ and substantial intrinsic scatter, suggesting that the relation is only beginning to emerge at $z\sim 5$. We measure a negative correlation between $f_{\rm DM}$ and baryonic surface density $\Sigma _{\rm bar}$, weaker but broadly consistent with trends at cosmic noon and at $z\sim 0$. Qualitatively comparing with modified NFW profiles coupled to an empirical stellar-to-halo mass relation suggests that the lowest $f_{\rm DM}$ ($\lesssim 0.4$) require cored inner DM profiles, while the highest fractions favour cuspier profiles, potentially reflecting adiabatic contraction. Overall, the elevated $f_{\rm gas}$ and $f_{\rm DM}$ at $z\gtrsim 4$ are compatible with progenitors of baryon-dominated systems at $z\sim 2$ and naturally anticipate overmassive black holes at fixed $M_{\star }$.