Glimmers in the Cosmic Dawn. III. On the Photometrically Determined Black Hole Mass to Stellar Mass Relation Across Cosmic Time

Alice R. Young, Matthew J. Hayes, Alberto Saldana-Lopez, Axel Runnholm, Vieri Cammelli, Jonathan C. Tan, Richard S. Ellis, Benjamin W. Keller, Jens Melinder, Jasbir Singh

First listed 2025-08-21 | Last updated 2025-08-21

Abstract

We present the results from performing spectral energy distribution (SED) fitting on 121 variable active galactic nuclei (AGN) candidates in the Hubble Ultra Deep Field (HUDF) using photometry from both the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) covering $0.2 - 4.8$ microns. We designed a bespoke SED fitting code which decomposes the total SED into its stellar and AGN contributions. Our SED fitting retrieves a significant contribution to the total SED from an AGN template for 26 of our variable sources with $0 < z < 7$. We leverage the model AGN spectrum to estimate black hole masses ($M_{BH}$) using the measured luminosity at 5100 Å and local empirical calibrations. Common with recently discovered JWST broad line AGN (BL-AGN), we observe a trend in the $M_{BH} - M_{\ast}$ plane where low redshift sources have $M_{BH}$ which agree with local relations while high redshift sources have increasingly overmassive black holes with respect to the stellar mass ($M_{\ast}$) of their host galaxies. Within our sample, we identify two IMBH candidates hosted by dwarf galaxies at $z<1$ featuring overmassive BHs in the $M_{BH}-M_{\ast}$ plane, similarly to our high redshift sources. Finally, our SED fitter successfully retrieves the AGN nature of one source at $z >6$. This object has $z_{phot} = 6.74$ and we estimate a lower limit on its black hole mass of $\log_{10}(M_{BH}/M_{\odot}) > 7.36$.

Short digest

Photometric SED decomposition of 121 variability-selected HUDF nuclei using HST+JWST (0.2–4.8 μm) isolates stellar and AGN components and estimates MBH from L5100 via local scalings. AGN contributions are significant in 26 sources across 0<z<7, including a zphot=6.74 object with a lower limit log10(MBH/M⊙)>7.36. In the MBH–M* plane, low‑z sources align with local relations while higher‑z systems—and two z<1 dwarf hosts flagged as IMBH candidates—are increasingly overmassive. A NIRISS‑confirmed BL‑AGN exemplifies successful recovery, with the AGN providing ≈95% of the flux in at least one band, underscoring accelerated early BH growth relative to hosts.

Key figures to inspect

• Figure 1 (completeness curves): Check the 90%‑recovery limits per filter from F480M PSF injections to gauge photometric depth and where AGN components might be missed in the faint regime.
• Figure 2 (source 316, z=4.61): Compare residuals and AIC between pure‑stellar and AGN+stellar fits to see a failure case where variability-selected AGN is not required by the SED—useful for understanding selection versus decomposition limits.
• Figure 3 (source 1807): Inspect the AGN+stellar fit that beats the pure‑stellar AIC and note the ≈95% AGN flux fraction in at least one band; cross‑check with NIRISS broad‑line confirmation as a method validation plot.
• Figure 4 (M* consistency): Examine M*(AGN+SP) versus M*(SP‑only) and versus redshift, color‑coded by maximum ΔmAB, to verify that stellar mass estimates are generally stable and not strongly biased by AGN inclusion.