Beyond the stars: Linking H α sizes, kinematics, and star formation in galaxies at z ≈ 4−6 with JWST grism surveys and geko

Year: 2026

Type: article

Abstract: 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 $\mathrm{H}\alpha$ 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 $\mathrm{H}\alpha$ 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 $z\approx 5$, we find that the average H $\alpha$ sizes are larger than the stellar continuum (FUV, NUV, and optical), with $r_{\rm e, \mathrm{H}\alpha } = 1.17 \pm 0.05$ kpc and $r_{\rm e,cont} \approx 0.9$ kpc for galaxies with $\log (M_{\star } \rm [{\rm M}_{\odot } ]) = 9.5$. 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 $r_{\rm e, \mathrm{H}\alpha }/r_{\rm e, NUV}$ increases with the distance above the star-forming main sequence ($\Delta \rm MS$), consistent with an expansion of H $\alpha$ 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 $\text{v}/\sigma _0$, which could be tracing accreting gas illuminated by the $\mathrm{H}\,\alpha$ emission. Finally, we find that about half of the elongated systems ($b/a\lt 0.5$) are not rotationally supported, indicating a potential flattened/prolate galaxy population at high redshift.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag437

Authors: A Lola Danhaive, Sandro Tacchella, William McClymont, Brant Robertson, Stefano Carniani, Courtney Carreira, Eiichi Egami, Andrew J Bunker, Emma Curtis-Lake, Daniel J Eisenstein, Zhiyuan Ji, Benjamin D Johnson, Marcia Rieke, Natalia C Villanueva, Christopher N A Willmer, Chris Willot, Zihao Wu, Yongda Zhu

Institutions: University of Cambridge, University of California, Santa Cruz, Scuola Normale Superiore, University of Arizona, University of Oxford, University of Hertfordshire, Center for Astrophysics Harvard & Smithsonian, The University of Texas at Austin, Herzberg Institute of Astrophysics

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $3,046

Funders: National Science Foundation, National Aeronautics and Space Administration, Entomological Society of America, University of California, Santa Cruz, University of Arizona, Space Telescope Science Institute, European Commission, Science and Technology Facilities Council, H2020 European Research Council

OpenAlex ID: W4415314643