Beyond the stars: Linking H$α$ sizes, kinematics, and star formation in galaxies at $zpprox 4-6$ with JWST grism surveys and $ exttt{geko}$

Understanding how galaxies assemble their mass during the first billion years of cosmic time is a central goal of extragalactic astrophysics, yet joint constraints on their sizes and kinematics remain scarce. We present one of the first statistical studies of the $\mathrm{H}α$ size-mass relation at high redshift with a sample of 213 galaxies at spectroscopic redshifts of $z\approx 4-6$ from the FRESCO and CONGRESS NIRCam grism surveys. We measure the $\mathrm{H}α$ morphology and kinematics of our sample using the novel forward modelling Bayesian inference tool $\texttt{geko}$, and complement them with stellar continuum sizes in the rest-frame FUV, NUV, and optical, obtained from modelling of imaging data from the JADES survey with $\texttt{Pysersic}$. At $z\approx5$, we find that the average H$α$ sizes are larger than the stellar continuum (FUV, NUV and optical), with $r_{\rm e, Hα}= 1.17 \pm 0.05$ kpc and $r_{\rm e,cont} \approx 0.9$ kpc for galaxies with $\log(M_{\star} ~\rm [M_{\odot}])= 9.5$. However, we find no significant differences between the stellar continuum sizes at different wavelengths, suggesting that galaxies are not yet steadily growing inside-out at these epochs. Instead, we find that the ratio $r_{\rm e, Hα}/r_{\rm e, NUV}$ increases with the distance above the star-forming main sequence ($Δ\rm MS$), consistent with an expansion of H$α$ sizes during episodes of enhanced star formation caused by an increase in ionising photons. As galaxies move above the star-forming main sequence, we find an increase of their rotational support $v/σ$, which could be tracing accreting gas illuminated by the \Ha\ emission. Finally, we find that about half of the elongated systems ($b/a < 0.5$) are not rotationally supported, indicating a potential flattened/prolate galaxy population at high redshift.

💡 Research Summary

This paper presents one of the first large‑scale statistical investigations of the H α size–mass relation and ionised‑gas kinematics for galaxies in the early Universe (redshift z ≈ 4–6). The authors combine deep JWST/NIRCam slitless‑spectroscopy from the FRESCO and CONGRESS surveys with multi‑band imaging from the JADES program. A sample of 213 spectroscopically confirmed galaxies is assembled; for each object the authors extract a continuum‑subtracted H α emission map from the NIRCam grism data (F444W for FRESCO, F356W for CONGRESS) and model the line profile with a forward‑modelling Bayesian inference tool called geko. The imaging data (F090W, F115W, F150W, F200W, F277W, F356W, F444W and several medium‑band filters) are fitted with the Sérsic‑profile code Pysersic to obtain rest‑frame FUV, NUV and optical half‑light radii, carefully correcting for the point‑spread function and pixel scale.

Key results: (1) The average H α half‑light radius at z ≈ 5 is rₑ, Hα = 1.17 ± 0.05 kpc, about 30 % larger than the stellar continuum radius (≈ 0.9 kpc) for galaxies with log M★/M⊙ = 9.5. (2) No statistically significant difference is found among the continuum sizes measured in FUV, NUV, and optical bands, indicating that at these epochs the UV‑to‑optical light is dominated by similarly young stellar populations (the “outshining” effect). (3) The ratio rₑ, Hα / rₑ, NUV rises monotonically with distance above the star‑forming main sequence (ΔMS), suggesting that episodes of enhanced star formation produce an expansion of the ionised‑gas region. (4) Galaxies with larger ΔMS also show higher rotational support (v/σ), implying that the bright H α emission traces rotating gas disks that become more dominant during star‑burst phases. (5) About half of the elongated systems (axis ratio b/a < 0.5) exhibit low v/σ (< 1), i.e., they are not rotation‑supported, hinting at a population of flattened or prolate structures at very high redshift.

Interpretation: The lack of wavelength‑dependent continuum size gradients argues against a well‑established inside‑out growth mode at z > 4; instead, galaxies appear to be in a bursty growth phase where star formation and gas accretion occur simultaneously across the whole galaxy. The correlation between H α size expansion and increased v/σ supports a picture where cold‑flow or other gas‑inflow mechanisms feed rotating disks that light up in H α. The presence of non‑rotating, highly elongated objects may reflect frequent mergers, anisotropic collapse, or early prolate morphologies predicted by cosmological simulations.

Methodologically, the study demonstrates that forward‑modelling of slitless grism data with a Bayesian framework can recover both spatial and kinematic information without the need for integral‑field spectroscopy. This opens the door for future work that combines geko results with NIRSpec IFU observations, metallicity maps, and higher‑resolution imaging to build a comprehensive view of how mass, size, dynamics, and star‑formation efficiency co‑evolve in the first billion years of cosmic history.