BlackTHUNDER Reveals a Massive Filament around a Compact AGN at $z\simeq 5.23$

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 $z\simeq 5.23$, 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]$\hat{I}»$5007 kinematics reveals a smooth large-scale velocity gradient centred on the central, massive ($M_{\star }\simeq 1.1\times 10^9$ $M_{\odot }$) and metal rich ($Z\sim 0.6$ $Z_{\odot }$) system of the group. In this source, only 2.4 kpc (projected) from GN-77652, [OIII]$\hat{I}»$4363 line diagnostics provide possible evidence for a second AGN. GN-77652 exhibits a shallow ($-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.

16 pages and 8 figures (main text). Submitted to A&A. Comments are welcome