A Selection Aware View of Black Hole-Galaxy Coevolution at High Redshift

Francesco Ziparo, Stefano Carniani, Simona Gallerani, Bartolomeo Trefoloni

First listed 2026-03-04 | Last updated 2026-03-04

Abstract

The large population of broad-line Active Galactic Nuclei (AGN) observed with the James Webb Space Telescope (JWST) at $z \gtrsim 4$ opens a new window onto the black hole-galaxy connection in the first Gyr of cosmic history. We use the JADES survey-level dataset and develop a forward-modeling Bayesian framework that explicitly accounts for broad H$α$ detectability, ensuring that selection effects are incorporated into the likelihood function. With this approach, we constrain the black hole-stellar mass ($M_{\mathrm{BH}}$-$M_\star$) relation to be $\log M_{\rm BH} = -4.06^{+0.50}_{-0.51} + 1.17^{+0.06}_{-0.06}\,\log M_\star$, with an intrinsic orthogonal scatter of $σ_{\rm int} = 0.63^{+0.14}_{-0.11}$ dex. The slope and normalization are consistent with local determinations, indicating that the average scaling was already established by $z \sim 4$-6. This suggests that the primary evolution of the relation occurs in its dispersion rather than in its mean normalization. In contrast, the substantially larger intrinsic scatter relative to the nearby Universe reveals a wider diversity of black hole-galaxy growth histories, likely driven by bursty accretion, delayed feedback, and differences in merger or seeding histories. Future JWST samples will be crucial to test whether this increased scatter is a persistent feature of the high-redshift Universe.

Short digest

Using JADES-level data, the authors build a forward-modeling Bayesian framework that folds broad Hα detectability directly into the likelihood, making the MBH–M⋆ inference selection-aware. They obtain log M_BH = −4.06(+0.50/−0.51) + 1.17(+0.06/−0.06) log M⋆ with an intrinsic orthogonal scatter σ_int = 0.63(+0.14/−0.11) dex at z ≈ 4–6. The slope and normalization match local relations, implying the average scaling was already in place early, while the substantially larger scatter signals more diverse growth paths. The takeaway is that apparent high‑z offsets are largely selection-driven, with primary evolution occurring in dispersion rather than mean normalization.

Key figures to inspect

• Figure 2 — Detectability maps across the MBH–M⋆ plane at two noise levels: inspect how the gold detection boundary shifts with sensitivity and which masses become recoverable; note where previous MBH–M⋆ relations (colored dashed lines) intersect the observable region.
• Figure 3 — Recovery tests: compare input local relations (green/red) to biased detected subsets and to fits with/without truncation (solid vs dotted blue) to see how the truncated-likelihood corrects selection bias and recovers the true scaling.
• Figure 4 — Main MBH–M⋆ result for the Juodžbalis et al. (2025a) sample: read off the best-fit relation, the large intrinsic scatter band, positions relative to Kormendy & Ho, Reines & Volonteri, and Pacucci, and how orange detectability curves and the Geris stack (red star) anchor the low-mass end.
• Figure 1 — Mock Hα line fits: examine single- vs double-Gaussian decompositions and residuals to understand how broad components are identified under realistic noise and why detectability depends on line width and contrast.