The dark side of early galaxies: $\text{geko}$ uncovers dark-matter fractions at $z\sim 4-6$

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 H$\alpha$ morpho-kinematics of 163 galaxies at redshift $z\approx 4$-6 from FRESCO and CONGRESS (with JADES imaging), using the $\text{geko}$ code, and infer rotational velocities and dispersions within $r_{\mathrm{e}}$. Our sample spans $\log M_{\star }\approx 7$-10 and $\log M_{\mathrm{dyn}}\approx 9$-11. Gas masses are estimated via scaling relations, yielding baryonic masses and dark-matter (DM) fractions $f_{\mathrm{DM}}(r<r_{\mathrm{e}})$ within the H$\alpha$ half-light radius. We find high median fractions of $⟨f_{\mathrm{gas}}⟩=0.77$ and $⟨f_{\mathrm{DM}}⟩=0.73$, where $f_{\mathrm{gas}}$ is measured with respect to the baryonic mass and $f_{\mathrm{DM}}$ with respect to the DM+baryonic mass. About two-thirds of systems are DM-dominated within $r_{\mathrm{e}}\sim 0.5-1$ kpc. Both $f_{\mathrm{gas}}$ and $f_{\mathrm{DM}}$ decrease with stellar mass, consistent with simulations. The stellar Tully-Fisher relation shows a tentative offset to higher $v_{\mathrm{circ}}$ 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 $f_{\mathrm{DM}}$ and baryonic surface density ${\Sigma }_{\mathrm{bar}}$, 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 $f_{\mathrm{DM}}$ ($\lesssim 0.4$) require cored inner DM profiles, while the highest fractions favour cuspier profiles, potentially reflecting adiabatic contraction. Overall, the elevated $f_{\mathrm{gas}}$ and $f_{\mathrm{DM}}$ at $z\gtrsim 4$ are compatible with progenitors of baryon-dominated systems at $z\sim 2$ and naturally anticipate overmassive black holes at fixed $M_{\star }$.

14 pages, 7 figures, 2 tables. Submitted to MNRAS

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