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

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 $\text{geko}$, 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 $\text{Pysersic}$. At $z\approx 5$, we find that the average H$\alpha$ sizes are larger than the stellar continuum (FUV, NUV and optical), with $r_{\mathrm{e},\mathrm{H}\alpha }=1.17\pm 0.05$ kpc and $r_{\mathrm{e},\mathrm{cont}}\approx 0.9$ kpc for galaxies with $\log (M_{\star }[{\mathrm{M}}_{\odot }])=9.5$. 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 $r_{\mathrm{e},\mathrm{H}\alpha }/r_{\mathrm{e},\mathrm{NUV}}$ increases with the distance above the star-forming main sequence ($\Delta \mathrm{MS}$), consistent with an expansion of H$\alpha$ 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 $v/\sigma$, 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.

18 pages, 11 figures, 2 tables, submitted to MNRAS

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