2024A&A...690A..64M

We present the first results on the spatial distribution of star formation in 454 star-forming galaxies just after the epoch of reionisation (4.8 < z < 6.5) using Hα emission-line maps and F444W imaging that traces the stellar continuum from the JWST FRESCO NIRCam Slitless Spectroscopy Survey. The Hα equivalent width profiles of star-forming galaxies across the main sequence at z ∼ 5.3 with stellar masses 6.8≤ log(M_*/M_⊙) < 11.1 increase with radius, which provides direct evidence for the inside-out growth of star-forming galaxies just after the epoch of reionisation. GALFIT was used to calculate half-light radii, R_eff, and central surface densities within 1 kiloparsec, Σ_1kpc of Hα and the continuum. At a fixed stellar mass of Log(M_*/M_⊙) = 9.5, Σ_1kpc, Hα_ is 1.04 ± 0.05 times higher than Σ_1kpc, C_, R_eff, Hα_ is 1.18 ± 0.03 times larger than R_eff, C_ and both R_eff measurements are smaller than 1 kiloparsec. These measurements suggest the rapid build-up of compact bulges via star formation just after the epoch of reionisation. By comparison to analogous work done at lower redshifts with Hubble Space Telescope WFC3 slitless spectroscopy as part of the 3D-HST (z ∼ 1) and CLEAR (z ∼ 0.5) surveys, we find that R_eff(z) evolves at the same pace for Hα and the continuum, but Σ_1kpc(z) evolves faster for Hα than the stellar continuum. As a function of the Hubble parameter, $ \frac{R_{\mathrm{eff, H}\alpha}}{R_{\mathrm{eff, C}}}={1.1h(z)} $ and $ \frac{\Sigma_{1\mathrm{kpc,H}\alpha}}{\Sigma_{1\mathrm{kpc,C}}}=h(z)^{{1.3}} $. These parametrisations suggest that the inside-out growth of the disk starts to dominate the inside-out growth of the bulge towards lower redshifts. This is supported by the redshift evolution in the EW(Hα) profiles from FRESCO, 3D-HST, and CLEAR at fixed stellar mass and when star-forming progenitors are traced, in which in EW(Hα) rapidly increases with radius within the half-light radius at z ∼ 5.3, but EW(Hα) increases only significantly with radius in the outer disk at z ∼ 0.5.