Undermassive Hosts of $z = 4-6 $ AGN from JWST/NIRCam Image Decomposition with CONGRESS, FRESCO, and JADES

TL;DR

The paper investigates the high-redshift MBH–M* relation by spatially decomposing the hosts of 17 faint broad-line AGN at z ≈ 4–6 using JWST/NIRCam data. Using GALFIT+MCMC, the authors separate AGN and host contributions across seven bands, detecting host emission in 9 targets and deriving host stellar masses that are frequently 1–2 dex lower than photometry-only SED results. They find MBH/M* ratios in the range 0.01–1.48, indicating black holes that are overdense relative to their hosts when compared to local scaling relations, though host sizes generally agree with established size–mass trends. The work underscores the necessity of imaging-based AGN–host decomposition to obtain robust host properties and highlights possible inside-out growth or suppressed star formation in these early systems, with implications for galaxy–black hole co-evolution models at high redshift.

Abstract

In the local Universe, supermassive black hole (SMBH) masses strongly correlate with their host-galaxies' stellar masses ($M_{*}$), but galaxies hosting faint AGN recently found by JWST may deviate from this relation. To constrain the M$_{\text{BH}}$-M$_{*}$ relation at high redshift, we performed AGN-host image decomposition for 17 low-luminosity AGN galaxies at $z$\,$\sim$\,4--6 using NIRCam images in the JADES GOODS-N field. These sources are identified as AGNs from broad H$α$ emission lines detected by the CONGRESS and FRESCO surveys. We used \textsc{galfit+MCMC} to fit spatial profiles in 7 wide-band images and detected extended emission in 9 sources out of 17. The close spatial alignment between the extended components and the AGN centers indicates that this emission likely originates from the host galaxies. These sources are extended at 0.9--2.0~$μ$m, suggesting significant host-galaxy light in the rest-frame UV. For the sources with the host detection, the stellar mass inferred based on image decomposition result can be 1-2 dex lower than the results without image decomposition. The BH-to-stellar mass ratio spans $M_{\text{BH}}/M_\ast$\,$\sim$\,0.01--1.48, placing them well above the local $M_{\text{BH}}$--$M_\ast$ relation. In contrast, the host-galaxy size--mass relation broadly agrees with previous measurements. Our results suggest that the host galaxies of these faint AGN are either genuinely under-massive compared to their black hole masses, or too compact to be spatially resolved.

Figures (12)

• Figure 1: False-color postage ($1.5\arcsec \times 1.5\arcsec$) stamps of the AGN-hosting galaxy sample from JWST/NIRCam cutouts. The RGB images are constructed by combining F090W+F115W (blue), F150W+F200W (green), and F277W+F356W+F444W (red). For GN1089568, GN1085355, GN1090549, and GN1087388, F090W, F115W, and the average of F356W and F444W are used for the blue, green, and red channels, respectively.
• Figure 2: Decomposition results of AGN galaxies
• Figure 3: Top panel: AGN-Host Decompostion of GN1014406. Cutout size is $1.5^{\prime\prime}\times 1.5^{\prime\prime}$. First column displays the orginal image in each band. Second column is the two component model (PSF + Sérsic). Third column shows the Sérsic model in each band. Fourth column show the AGN-subtracted image, where in SW bands we can find some extended emission left as a signal of host detection. Fifth column is the residue image after subtracting both PSF model and Sérsic model, with reduced $\chi^2$ attached to each image. Sixth column shows the residue image when we attempt to fit the original image with one single PSF model, with reduced $\chi^2$ attached. The last column displays the radial brightness profile of original data (black), Sérsic model (red), AGN model (blue). Bottom panel: SED of GN1014406. The blue solid curve shows the total flux. The orange dashed curve shows the host flux measured from the AGN-subtracted image. The green dash--dotted and red dotted curves show the flux of point-source and Sérsic models, respectively. The shaded regions indicate the $1\sigma$ uncertainties. The full set of fitting results for all targets is available as a figure set.
• Figure 4: Photometry from Imaging decomposition.
• Figure 5: SED fits for four selected targets. Green curves show the AGN component, orange curves the stellar component, and blue curves the total model flux. Solid lines correspond to results based on image decomposition. Model-predicted fluxes are shown as symbols: blue circles for the total flux, orange squares for the stellar component, and green diamonds for the AGN component. Dashed lines indicate photometry-based SED fitting performed without image decomposition. The top two panels present cases in which AGN signatures are clearly detected in the photometry-based SED fits, while the bottom two panels show sources for which the photometry-based SED fitting alone does not clearly identify an AGN component. The full set of SED fitting results for all targets is available as a figure set.