Spatially Resolved Active Galactic Nucleus Ionization and Star Formation at Cosmic Noon with JWST/JEMS

Abstract

At cosmic noon (z ≈ 2–3), both star formation and active galactic nucleus (AGN) activity peaked, each playing a significant role in ionizing interstellar gas on galaxy-wide scales. The spatial distribution of this ionized gas provides a direct probe of how AGN and stellar ionization influence the gaseous reservoirs of their host galaxies. Using JWST/NIRCam imaging from the JWST Extragalactic Medium-band Survey (JEMS), we spatially map two complementary tracers of ionized gas, [O iii]+Hβ and Paβ, in ∼200 galaxies at 2.5 < z < 2.9. We apply multiwavelength AGN diagnostics to divide the sample into AGN hosts (33 galaxies), Paβ-detected systems (32 galaxies), and control objects (175 galaxies). We measure the [O iii]+Hβ and Paβ spatial extents in each population and relate them to AGN and host properties derived from spectral energy distribution (SED) modeling. Both tracers exhibit systematically larger maximum radial extents in AGN hosts than in control galaxies (by ∼0.3 dex), with [O iii]+Hβ emission modestly more extended than Paβ by ∼0.1 dex. With this statistically robust AGN sample, we measure the [O iii]+Hβ radial extent–AGN luminosity relation at z ∼ 3 and derive a slope of ∼0.2, consistent with the shallow end of values reported at low redshift. The larger ionized gas extents among AGN hosts relative to the control sample, combined with the strong correlation between [O iii]+Hβ extent and AGN luminosity, suggest that AGN activity may dominate gas ionization in galaxies with mixed AGN and star-forming activity at cosmic noon, although stellar processes can still contribute significantly on kiloparsec scales.