AGN--Host Galaxy Image Decomposition with JWST

TL;DR

The paper tackles the problem of disentangling AGN light from host galaxies in deep JWST imaging, a task complicated by PSF modeling and fitting software. It compares PSFEx and photutils PSFs and fits with Galfit and AstroPhot on 87 CEERS X-ray AGNs, highlighting that PSF choice strongly affects AGN magnitudes while the Sérsic-based host parameters $n$ and $r_e$ are highly degenerate and depend on the fitting tool. The study finds that Sérsic profiles do not uniquely describe typical AGN hosts in extragalactic surveys, with non-Sérsic features (bars, arms) contributing substantially to the radial light profile and limiting the interpretability of $n$. The authors propose practical recommendations, including using photutils-derived PSFs, adopting AstroPhot for fits, and supplementing with model-independent morphology measurements after AGN subtraction to enable robust cross-study comparisons. Together, these findings inform best practices for future AGN host studies with JWST CEERS and other deep surveys.

Abstract

The ability to disentangle the light of an AGN from its host galaxy is strongly dependent on the spatial resolution and depth of the imaging. As the capabilities of imaging systems improve with time, confirming that our standard techniques adequately model the increasingly complex structures unveiled is essential. With JWST providing unprecedented image quality, we can test how measurements of galaxy morphology vary with the choice of point-spread function (PSF) and fitting software. We perform two-component Sérsic+PSF fits of the surface brightness profiles of 87 X-ray AGNs $(0.1 < z < 4)$ from the CEERS survey. We create model PSFs for NIRCam F115W imaging using both photutils and PSFEx. We find that PSFEx models consistently fail to match the radial profile of typical point sources within our sample. We then perform AGN--host decompositions on each source by creating Sérsic+PSF models using both Galfit and AstroPhot. We find that Galfit and AstroPhot converge to different regions of the parameter space, providing consistently differing host galaxy properties. While we can measure the AGN and host magnitudes accurately, we find that the host galaxy morphological parameters are not well-determined -- the Sérsic index and effective radius are strongly covariant. Significant changes in the host galaxy parameters do not correspond to changes in the statistical quality of fit, nor to significant changes in the model's radial profile. These results indicate that the Sérsic profile does not uniquely well-represent typical AGN host galaxies in extragalactic survey fields. We also provide recommendations for studies of AGN hosts comparable to ours.

Figures (11)

• Figure 1: The rest-frame X-ray luminosity in the 210 band from Nandra2015 as a function of redshift. Sources in red and blue are included in our sample, whereas the gray and dark gray sources are not contained in our sample. Round markers are sources with a confident spectroscopic redshift, whereas crosses indicate photometric redshifts from multiband SED fitting of up to 35 bands of UV to mid-IR imaging. The dashed line represents the X-ray flux limit converted to luminosity. The X-ray luminosities have been K-corrected to rest-frame 2--10 keV.
• Figure 2: Example of a poorly behaved HST/ACS F814W point-source candidate (right) alongside a well-behaved candidate (left). Each cutout shows the central $2\arcsec\times2\arcsec$ region of the point source. The poorly behaved point source shows several anomalous pixels on the upper region of the core; these effects can significantly contaminate any PSF model constructed using the point source.
• Figure 3: The distribution of the FWHM of each PSF model and their constituent point-source sample for JWST (top panel) and HST (bottom panel) as measured by AstroPhot. The FWHM for each is normalized by the median FWHM of the Global point source sample. The black points represent each point source within the sample. The gray box-and-whisker plots provide information on the distribution of the point sources: the central line in the box gives the median value, the extent of the box gives the quartiles above and below, and the whiskers include the rest of the sample. The red and blue points represent the PSF models for photutils and PSFEx, respectively. The green stars are the PSF FWHM values from Zhuang2024; only four of our JWST groupings overlap their study.
• Figure 4: An example of a photutils (left) and PSFEx (middle) PSF for JWST/NIRCam's F115W filter. Each cutout is 4 in diameter. This particular example features the Global grouping. The rightmost plot shows the 1D radial profile of each PSF (photutils in red, PSFEx in blue) alongside their constituent point sources in gray. The dashed lines represent the Gaussian fit of the PSF core we used to determine the PSF FWHM.
• Figure 5: Comparison of the central 0.825$\times$0.825 region of the Dither 2A grouping's PSFs for JWST/NIRCam F115W. The photutils and PSFEx models are the left and middle panels, respectively. We also include the publicly available PSFEx model generated by Zhuang2024 in the right panel. All three models are normalized to the same log scale.