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

TL;DR

This study uses JWST/NIRCam grism surveys to statistically map the Hα size–mass relation at z ≈ 4–6 for 213 galaxies, comparing Hα morphologies with rest-frame UV–optical continuum sizes derived from multi-band imaging. By forward-modeling Hα emission with geko and continuum light with pysersic, the authors quantify SMRs across wavelengths, finding larger Hα extents (≈1.17 kpc at log(M*/M⊙)=9.5) than continua (≈0.9 kpc), with a shallower Hα slope (≈0.15) and weak redshift evolution. The multi-wavelength analysis reveals that Hα–UV size ratios grow with distance above the star-forming main sequence, consistent with burst-driven Strömgren spheres rather than steady inside-out growth, and that Hα sizes correlate with rotational support only for rising SFHs, implying a link to the baryon cycle. Additionally, a significant fraction of elongated systems are not rotationally supported, indicating flattened/prolate high-redshift morphologies. These results constrain galaxy growth models by connecting nebular-scale star formation, ionized gas kinematics, and morphological evolution in the early universe, while highlighting the importance of forward-modeling and larger, diverse samples to fully interpret high-z galaxy structure.

Abstract

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.

Figures (16)

• Figure 1: Stellar mass ($M_{\star}$) as a function of spectroscopic redshift ($z_{\rm spec}$) for our sample of 213 galaxies, colour-coded by sSFR, average over 10 Myr. As expected, we are biased to high sSFR systems at low stellar masses, as these systems will be the brightest in $\mathrm{H}\alpha$ and pass our S/N cut.
• Figure 2: From top to bottom: rest-frame FUV, NUV and optical pysersic fits for a galaxy in our sample, probed by the NIRCam filters F090W, F200W, and F356W at $z\approx5.2$. In each band, the galaxy (left) is modelled with a PSF (red circle)-convolved one-component Sérsic profile (middle), and contaminants are masked during the fitting. The residual images (right) show signs of a clumpy morphology which is not fully captured by this simple model. However, the overall size (effective radius) of the galaxy is well measured by the model.
• Figure 3: Size-mass relation at $z=4-6$ for sizes measured in $\mathrm{H}\alpha\xspace$ and rest-frame NUV, FUV, and optical for the galaxies in our sample (circles) colour-corded by their offset from the main sequence $\Delta \rm MS\xspace$. We fit the SMR (Eq. \ref{['eq:SMR']}) for the galaxies above $\log(M_{\star}\xspace ~\rm [M_{\odot}\xspace])\xspace>9$, where our sample is representative, and plot the best-fit SMR at $z=5$ (red line). We also show our fit using the full sample (purple dashed line). The shaded regions for both fits show the uncertainties. We compare it to relations presented in the literature Miller:2024aaAllen:2025aaYang:2025aa, as well as prediction from cosmological simulations (thesan; Shen:2024aa, thesan-zoom; McClymont:2025ab, tng50; Costantin:2023aa, and flares; Roper:2022aa).
• Figure 4: Average half-light ratio of $\mathrm{H}\alpha\xspace$ emission (top), FUV (middle), and optical continuum (bottom) for a $\log_{10}(M_{\star}\xspace/M_{\odot}\xspace)\xspace=9.5$ galaxy as a function of redshift for the galaxies in this work (red circles). In all cases we find flat slopes for the redshift evolution (red lines; see Tab. \ref{['tab:size-fits']}). This is most likely due to the small redshift range probed, but could point to a flattening at high redshift. We compare with other works across cosmic time Shibuya:2015aaWilman:2020aaNedkova:2021aaMatharu:2024aaMiller:2024aaWard:2024aaAllen:2025aaYang:2025aa, who typically find negative slopes. This is also consistent with predictions from the thesanShen:2024aa, thesan-zoomMcClymont:2025ab, tng50Costantin:2023aa, and flaresRoper:2022aa simulations.
• Figure 5: Evolution of the best-fit slope $\alpha$ of the mass dependence (top; Eq. \ref{['eq:SMR']}) and intrinsic scatter $\sigma_{\rm int}$ (bottom) for the $\mathrm{H}\alpha$, FUV, NUV, and optical sizes (left to right) for our sample with (red shaded regions) and without (purple shaded regions) applying a $\log(M_{\star}\xspace ~\rm [M_{\odot}\xspace])\xspace>9$ mass cut. We fit our full sample with a constant slope and scatter. We compare our measurements to other parametric fits from observations van-der-Wel:2014abMowla:2019ab and the thesan-zoom simulation McClymont:2025ab, as well as results from fits in redshift bins across cosmic time Shibuya:2015aaMatharu:2024aaMiller:2024aaVaradaraj:2024aaWard:2024aaAllen:2015aaYang:2025aa.