BlackTHUNDER Reveals a Massive Filament around a Compact AGN at $z\simeq5.23$

Giulia Tozzi, Hannah Übler, Eleonora Parlanti, Roberto Maiolino, Claudia Pulsoni, Rachel Somerville, Mirko Curti, Giovanni Mazzolari, Capucine Barfety, Elena Bertola, Andrew J. Bunker, Stefano Carniani, Giovanni Cresci, Richard Davies, Francesco D'Eugenio, Frank Eisenhauer, Natascha M. Förster Schreiber, Reinhard Genzel, Lucy R. Ivey, Ignas Juodžbalis, Dieter Lutz, Cosimo Marconcini, Thorsten Naab, Meghana Pannikkote, Stavros Pastras, Michele Perna, Letizia Scaloni, Raffaella Schneider, Linda J. Tacconi, Giacomo Venturi

First listed 2026-07-08 | Last updated 2026-07-06

Abstract

Despite the growing number of compact active galactic nuclei (AGN) at $z>4$ discovered by JWST, their formation and evolution remain poorly understood. This paper investigates the large-scale environment of GN-77652, a compact AGN at $z=5.229$ observed as part of the JWST NIRSpec IFU Large Program BlackTHUNDER and complemented by deep multi-band NIRCam imaging. GN-77652 lies in close proximity to a 12 kpc-long filament composed of multiple sources at $z\simeq5.23$, spanning a remarkable range in stellar masses ($M_{\star}=0.7-13 \times 10^8$ ${M_\odot}$), gas phase metallicities (12$+$log(O/H) $=$ 7.6-8.5) and star formation rates (SFR $=0.4-6$ ${M_\odot}$ yr$^{-1}$). The [OIII]$λ$5007 kinematics reveals a smooth large-scale velocity gradient centred on the central, massive ($M_{\star}\simeq1.1\times10^9$ ${M_\odot}$) and metal rich ($Z\sim0.6$ $Z_{\odot}$) system of the group. In this source, only 2.4 kpc (projected) from GN-77652, [OIII]$λ$4363 line diagnostics provide possible evidence for a second AGN. GN-77652 exhibits a shallow ($-30$ to $+20$ km s$^{-1}$) velocity gradient that is consistent with disk rotation according to dynamical modelling. The Lyman-Werner radiation field produced by the filament is too weak for the black hole (BH) in GN-77652 to have formed recently via direct collapse. However, the required conditions may have existed at earlier epochs, or alternative scenarios (e.g. a recoiling BH ejected from the filament) could also be plausible. The whole system is expected to coalesce in $150-440$ Myr, also motivating an exploration of its future evolution through toy-model extrapolations and numerical simulations. Our analysis suggests that the compact AGN appearance of GN-77652 represents a transient evolutionary phase, consistent with the apparent decline with redshift in number density of compact AGN identified with JWST.

Short digest

Using JWST/NIRSpec IFU data from BlackTHUNDER plus deep NIRCam imaging, this paper maps the z=5.229 environment of the compact broad-line AGN GN-77652 and shows that it sits beside a 12 kpc filament of multiple galaxies at the same redshift. The filament spans nearly an order of magnitude in stellar mass, metallicity, and star-formation rate, and its [O III] λ5007 kinematics trace a smooth large-scale velocity gradient centered on a massive, metal-rich source where [O III] λ4363 diagnostics also suggest a second AGN just 2.4 kpc in projection from GN-77652. In contrast, GN-77652 itself shows only a shallow -30 to +20 km s^-1 gradient consistent with disk rotation, while the full complex is likely to coalesce within about 150-440 Myr. The payoff is a concrete picture of a compact JWST-selected AGN as a transient phase in a dense assembling system, with the local Lyman-Werner field too weak to favor very recent direct-collapse black-hole formation.

Key figures to inspect

• Figure 2. Use this as the system-definition figure. It isolates GN-77652 and the four filament members in [O III], then pairs that morphology with integrated spectra showing that the components have genuinely different continua and line ratios, which is essential for the paper's claim that this is a multi-source filamentary complex rather than a single disturbed emitter.
• Figure 3. This is the key environmental kinematics figure. The [O III] velocity field shows the large-scale gradient centered on source B across the filament, while the inset makes clear that GN-77652 itself has only a much shallower velocity gradient, motivating the paper's contrast between filament-scale assembly and a comparatively ordered compact AGN host.
• Figure 4. This figure carries the paper's rotation claim for GN-77652. The data-model-residual comparison and the offset between the fitted position angle and the cross-slice direction are the evidence the authors use to argue that the detected gradient is consistent with disk rotation and is not an instrumental artifact.
• Figure 6. This is the decisive ionization-diagnostic figure. The standard high-redshift BPT-like panels are not conclusive, but the [O III] λ4363 auroral-line diagnostics place both GN-77652 and the nearby massive source B in the AGN-like regime, which is why the paper raises the possibility of a second AGN only 2.4 kpc away.
• Figure 8. Use this for the bottom-line evolutionary interpretation. By evolving the summed z=5.23 complex forward in stellar and black-hole growth and comparing it with simulations and broader samples, the figure visualizes the idea that GN-77652 is a transient compact-AGN phase within a system expected to merge into a more massive descendant.