The dark side of early galaxies: $\texttt{geko}$ uncovers dark-matter fractions at $z\sim4-6$
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| Format: | Preprint |
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2025
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| author | Danhaive, A. Lola Tacchella, Sandro Bunker, Andrew J. Curtis-Lake, Emma de Graaff, Anna D'Eugenio, Francesco Duan, Qiao Egami, Eiichi Eisenstein, Daniel J. Johnson, Benjamin D. Maiolino, Roberto McClymont, William Rieke, Marcia Robertson, Brant Sun, Fengwu Willmer, Christopher N. A. Wu, Zihao Zhu, Yongda |
| author_facet | Danhaive, A. Lola Tacchella, Sandro Bunker, Andrew J. Curtis-Lake, Emma de Graaff, Anna D'Eugenio, Francesco Duan, Qiao Egami, Eiichi Eisenstein, Daniel J. Johnson, Benjamin D. Maiolino, Roberto McClymont, William Rieke, Marcia Robertson, Brant Sun, Fengwu Willmer, Christopher N. A. Wu, Zihao Zhu, Yongda |
| contents | 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 $M_{\star}$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_14779 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | The dark side of early galaxies: $\texttt{geko}$ uncovers dark-matter fractions at $z\sim4-6$ Danhaive, A. Lola Tacchella, Sandro Bunker, Andrew J. Curtis-Lake, Emma de Graaff, Anna D'Eugenio, Francesco Duan, Qiao Egami, Eiichi Eisenstein, Daniel J. Johnson, Benjamin D. Maiolino, Roberto McClymont, William Rieke, Marcia Robertson, Brant Sun, Fengwu Willmer, Christopher N. A. Wu, Zihao Zhu, Yongda Astrophysics of Galaxies 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 $M_{\star}$. |
| title | The dark side of early galaxies: $\texttt{geko}$ uncovers dark-matter fractions at $z\sim4-6$ |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2510.14779 |