BlackTHUNDER Reveals a Massive Filament around a Compact AGN at z 5.23

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 5.23, spanning a remarkable range in stellar masses (M=0.7-13 times 10 8 M), gas phase metallicities (12 + log(O/H) = 7.6-8.5) and star formation rates (SFR =0.4-6 M yr-1). The [OIII] lambda 5007 kinematics reveals a smooth large-scale velocity gradient centred on the central, massive (M 1.1 times10 9 M) and metal rich (Z about0.6 Z) system of the group. In this source, only 2.4 kpc (projected) from GN-77652, [OIII] lambda 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.