The Stellar Morphology and Size of X-Ray-selected Active Galactic Nucleus Host Galaxies Revealed by JWST

Abstract We investigate the stellar shape and size–mass relationship of X-ray-selected active galactic nucleus (AGN) host galaxies using the high angular resolution and deep sensitivity in the near-infrared of the COSMOS-Web JWST survey field. We present the rest-frame 1 μ m size, stellar mass, Sérsic index, axis ratio, and Gini– M 20 parameters of 690 moderate-luminosity active galactic nuclei (AGNs) between redshift 0 and 3 and with stellar mass <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>s</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>10.75</mml:mn> </mml:math> . We find that AGN host galaxies have effective radii of 1–5 kpc, which is in between star-forming galaxies (SFGs) and quiescent galaxies (QGs) of the same stellar mass. AGN hosts have similar size–mass trends to SFGs and QGs, being smaller at higher redshift for the same stellar mass. The slope of the size–mass relationship of AGN host galaxies is steeper than that of SFGs. Their rest-frame 1 μ m stellar morphology indicates a significant spheroidal component. We observed a low merger fraction (6%) in our sample, as well as substructures similar to disks, bars, and spiral arms in the residual images, which are in tension with evolutionary pathways that require major mergers. However, they may also be due to the different timescales between mergers and AGN activity.