Beyond the stars: Linking H α sizes, kinematics, and star formation in galaxies at z ≈ 4−6 with JWST grism surveys and 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 size–mass relation at high redshift with a sample of 213 galaxies at spectroscopic redshifts of from the FRESCO and CONGRESS NIRCam grism surveys. We measure the morphology and kinematics of our sample using the novel forward modelling Bayesian inference tool geko, and complement them with stellar continuum sizes in the rest-frame far ultraviolet (FUV), near ultraviolet (NUV), and optical, obtained from modelling of imaging data from the JADES survey with Pysersic. At , we find that the average H sizes are larger than the stellar continuum (FUV, NUV, and optical), with kpc and kpc for galaxies with . 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 increases with the distance above the star-forming main sequence (), consistent with an expansion of H sizes during episodes of enhanced star formation caused by an increase in ionizing photons. As galaxies move above the star-forming main sequence, we find an increase of their rotational support , which could be tracing accreting gas illuminated by the emission. Finally, we find that about half of the elongated systems () are not rotationally supported, indicating a potential flattened/prolate galaxy population at high redshift.