Sophie Lebowitz

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 ]+H β and Pa β , in ∼200 galaxies at 2.5 < z < 2.9. We apply multiwavelength AGN diagnostics to divide the sample into AGN hosts (33 galaxies), Pa β -detected systems (32 galaxies), and control objects (175 galaxies). We measure the [O iii ]+H β and Pa β 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 ]+H β emission modestly more extended than Pa β by ∼0.1 dex. With this statistically robust AGN sample, we measure the [O iii ]+H β 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 ]+H β 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.

The extended narrow-line region (NLR) of active galactic nuclei (AGNs) provides a valuable laboratory for exploring the relationship between AGNs and their host galaxies, often appearing as an “ionization cone” that can extend out to the very edge of the galaxy. We use new James Webb Space Telescope NIRCam imaging to study the morphologies and sizes of NLRs traced by [O iii] at cosmic noon (z ∼ 2–3). To determine NIRCam’s ability to probe the properties of the NLR in cosmic noon galaxies, we present simulated images of AGNs at z = 2–3 created with archival data cubes from the Multi Unit Spectroscopic Explorer of nine nearby (z < 0.05) AGN host galaxies with previously confirmed extended NLRs. Our simulated images show that NIRCam is able to resolve the morphologies of NLRs at z = 2–3 with narrow- and medium-band observations. We then search for extended NLRs with NIRCam medium-band observations targeting the [O iii] + Hβ emission in 27 previously identified AGNs at z = 2.4–3.4 in the Great Observatories Origins Deep Survey South field. We detect six galaxies in our observed sample with [O iii] + Hβ morphologies consistent with AGN ionization cones with characteristic NLR sizes of 1–2.5 kpc. Thanks to our simulated data, we can predict the effects of cosmological surface brightness dimming and instrument noise on the NLR size measurements at high redshift, which has the biasing effect of lowering the NLR size–AGN luminosity trend that is observed at low redshift by a factor of ∼2.