The dark side of early galaxies: $\texttt{geko}$ uncovers dark-matter fractions at $z\sim4-6$

A. Lola Danhaive, Sandro Tacchella, Andrew J. Bunker, Emma Curtis-Lake, Anna de Graaff, Francesco D'Eugenio, Qiao Duan, Eiichi Egami, Daniel J. Eisenstein, Benjamin D. Johnson, Roberto Maiolino, William McClymont, Marcia Rieke, Brant Robertson, Fengwu Sun, Christopher N. A. Willmer, Zihao Wu, Yongda Zhu

First listed 2025-10-16 | Last updated 2025-10-16

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$α$ morpho-kinematics of 163 galaxies at redshift $z\approx4$-6 from FRESCO and CONGRESS (with JADES imaging), using the $\texttt{geko}$ code, and infer rotational velocities and dispersions within $r_{\rm e}$. Our sample spans $\log M_{\star}\approx7$-10 and $\log M_{\rm dyn}\approx9$-11. Gas masses are estimated via scaling relations, yielding baryonic masses and dark-matter (DM) fractions $f_{\rm DM}(r<r_{\rm e})$ within the H$α$ half-light radius. We find high median fractions of $\langle f_{\rm gas}\rangle=0.77$ and $\langle f_{\rm DM}\rangle=0.73$, where $f_{\rm gas}$ is measured with respect to the baryonic mass and $f_{\rm DM}$ with respect to the DM+baryonic mass. About two-thirds of systems are DM-dominated within $r_{\rm e}\sim0.5-1$ kpc. Both $f_{\rm gas}$ and $f_{\rm DM}$ decrease with stellar mass, consistent with 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 $z\sim5$. We measure a negative correlation between $f_{\rm DM}$ and baryonic surface density $Σ_{\rm bar}$, weaker but broadly consistent with trends at cosmic noon and at $z\sim0$. Qualitatively comparing with modified NFW profiles coupled to an empirical stellar-to-halo mass relation suggests that the lowest $f_{\rm DM}$ ($\lesssim0.4$) 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 $z\gtrsim4$ are compatible with progenitors of baryon-dominated systems at $z\sim2$ and naturally anticipate overmassive black holes at fixed $M_{\star}$.

Short digest

Models Hα morpho-kinematics from JWST/NIRCam slitless spectra for 163 z≈4–6 star-forming galaxies (FRESCO+CONGRESS with JADES imaging) using the geko code to infer v_circ, dispersions, and M_dyn within r_e. With gas masses from scaling relations, the sample shows very high median fractions f_gas≈0.77 (of baryons) and f_DM≈0.73 (of total), with ≈2/3 DM‑dominated inside r_e≈0.5–1 kpc; both fractions decline with stellar mass. The stellar Tully–Fisher plane is offset to higher v_circ at fixed M⋆ with large intrinsic scatter, and f_DM anti‑correlates with baryonic surface density Σ_bar. Interpreting the spread with modified NFW profiles points to cored halos at the lowest f_DM and adiabatic contraction at the highest, implying these z≳4 systems evolve into the baryon‑dominated population by z≈2 and naturally anticipate overmassive BHs at fixed M⋆.

Key figures to inspect

• Figure 1: Inspect how far most points fall below the M_dyn=M⋆ line and whether sSFR offset tracks that displacement, to gauge the non‑stellar (gas+DM) share and identify the few 1:1 outliers.
• Figure 2: Read the fitted Tully–Fisher zero‑point and intrinsic scatter; verify the low‑mass drop‑off and compare directly to KMOS3D, RC100, and z≈0 curves for the emerging relation at z~5.
• Figure 3: Quantify the power‑law decline of f_gas and f_DM with M⋆ and the characteristic uncertainties; check agreement with thesan‑zoom and TNG50 medians and note where scaling‑relation errors dominate scatter estimates.
• Figure 4: Examine the steep negative f_DM–Σ_bar trend and its offset from cosmic‑noon relations; use the mass and f_gas color‑coding to see how compactness and gas richness drive DM dominance, and compare to ATLAS3D and DiskMass anchors.