The dawn of discs: unveiling the turbulent ionized gas kinematics of the galaxy population at z ∼ 4–6 with JWST/NIRCam grism spectroscopy

ABSTRACT Recent studies of gas kinematics at high redshift have reported discy systems that appear to challenge models of galaxy formation, but it is unclear whether they are representative of the underlying galaxy population. We present the first statistical sample of spatially resolved ionized gas kinematics at high redshift, comprised of 213 H $\alpha$ emitters in GOODS-S and GOODS-N at redshifts $z\approx 3.9\!-\!6.5$, observed with James Webb Space Telescope/NIRCam slitless spectroscopy and imaging from JADES, FRESCO, and CONGRESS. The sample probes two orders of magnitude in stellar mass ($\log (M_{\star }[\mathrm{M}_{\odot }])\approx 8\!-\!10$) and star formation rate ($\text{SFR}\approx 0.3\!-\!100\, \mathrm{M}_{\odot }\, \mathrm{yr}^{-1}$), and is representative down to $\log (M_{\star }[\mathrm{M}_{\odot }])\approx 9$. Using a novel inference tool, geko, we model the grism data to measure morphological and kinematic properties of the ionized gas, as probed by H $\alpha$. Our results are consistent with a decrease of the rotational support $v/\sigma _0$ and increase of the velocity dispersion $\sigma _0$ with redshift, when compared to $z< 3$, with $\sigma _0\approx 100$ km s$^{-1}$ and $v/\sigma _0\approx 1\!-\!2$ at $z\approx 3.9\!-\!6.5$. We study the relations between $\sigma _0$ and $v/\sigma _0$, and different star formation tracers and find a large scatter and diversity, with the most significant correlation between $\sigma _0$ and SFR. We find no evolution of the fraction of rotationally supported systems ($v/\sigma _0>1$) from $z\sim 5.5$ to $z\sim 4.5$, measured at $f=(34\pm 5){{\ \rm per\ cent}}$ in both redshift bins, for galaxies with masses $9<\log (M_{\star }[\mathrm{M}_{\odot }])< 10$. Overall, discs do not dominate the turbulent high-redshift galaxy population in the mass range probed by this work, but they remain a sizeable population. When placed in the context of studies up to cosmic noon, our results are consistent with a needed increase of disc-like systems with cosmic time.