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

Zheng Ma, Eichi Egami, Yongda Zhu, Fengwu Sun, Jianwei Lyu, Junyu Zhang, Christopher N. A. Willmer, Andrew J. Bunker, Stefano Carniani, Emma Curtis-Lake, Ryan Hausen, Xihan Ji, Zhiyuan Ji, Ignas Juodžbalis, Roberto Maiolino, George H. Rieke, Pierluigi Rinaldi, Yang Sun, Sandro Tacchella, Hannah Übler, Christina C. Williams

First listed 2026-01-22 | Last updated 2026-01-22

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.

Short digest

NIRCam image decomposition of 17 broad Hα–selected, low-luminosity AGN at z≈4–6 in JADES GOODS-N (CONGRESS+FRESCO) isolates host emission in 9/17 sources across 0.9–2.0 μm using GALFIT+MCMC. For those with detected hosts, stellar masses from decomposition-informed SEDs are 1–2 dex lower than photometry-only fits, yielding extreme BH-to-stellar mass ratios of M_BH/M_* ≈ 0.01–1.48—well above the local relation. Despite this, host size–mass measurements broadly align with established trends. The results point to genuinely under-massive hosts or hosts so compact they remain unresolved, underscoring strong early BH growth relative to their galaxies.

Key figures to inspect

• Figure 1 (NIRCam RGB cutouts): Scan GN1014406 and the GN1089xxx/GN1087xxx objects to see compact nuclei with faint, aligned extended structure; note the alternative RGB construction for GN1089568, GN1085355, GN1090549, GN1087388 and how rest-UV light appears despite red colors.
• Figure 2 (GN1014406 decomposition): Use the AGN-subtracted SW-band panels and radial profiles to confirm resolved host flux and improved residuals versus a PSF-only fit (compare reduced χ² columns); this illustrates the methodology behind host detections across the sample.
• Figure 3 (SED fits, 4 examples): Compare solid (with decomposition) vs dashed (total-photometry-only) models to see how adding AGN+host separation lowers M_* and reveals AGN components in cases where photometry alone missed them; inspect the partition of flux between AGN (green) and stars (orange).
• Figure 4 (Stellar mass comparison): Track object-by-object shifts relative to Zhang et al. 2025 via the dashed connectors; the blue/orange coding shows which sources exhibit AGN signatures in photometry-only fits, quantifying the typical 1–2 dex M_* reductions.