A first look at spatially resolved star formation at 4.8 < z < 6.5 with JWST FRESCO NIRCam slitless spectroscopy

¹ Cosmic Dawn Center, Copenhagen, Denmark
² Niels Bohr Insitute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
³ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO 80309, USA
⁴ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
⁶ MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, 02139 Massachusetts, USA
⁷ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, Texas 78712, USA
⁸ Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
⁹ Leiden Observatory, Leiden University, NL-2300 RA, Leiden, Netherlands
¹⁰ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800 9700 AV, Groningen, The Netherlands
¹¹ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
¹² Department of Physics & Astronomy, Tufts University, MA 02155, USA
¹³ Department of Physics and Astronomy, University of California Riverside, 900 University Avenue, Riverside, CA 92521, USA
¹⁴ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK

Received: 26 April 2024
Accepted: 18 July 2024

Abstract

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, R_eff, Hα/R_eff,C = 1.1h(z) and Σ_{1 kpc, Hα}/Σ_1 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.