2024A&A...687A.283S

Recent observations with the James Webb Space Telescope (JWST) have further refined the spectroscopic redshift of GN-z11, one of the most distant galaxies identified with the Hubble Space Telescope, at z = 10.603. The presence of extremely dense gas (> 10¹⁰ cm^–3), the detection of high-ionisation lines and of CII*1335 emission, and the presence of an ionisation cone indicate that GN-z11 also hosts an active galactic nucleus. Further photometric and spectroscopic follow-up demonstrates that it lies in a large-scale, overdense structure with possible signatures of Population III stars in its halo. Surprisingly, Lyα has also been detected despite the expected largely neutral intergalactic medium at such a redshift. We exploit recent JWST/NIRSpec integral field unit observations to demonstrate that the Lyα emission in GN-z11 is part of an extended halo with a minimum size of 0.8-3.2 kpc, depending on the definition used to derive the halo size. The surface brightness of the Lyα halo around GN-z11 appears consistent with Lyα halos observed around z ∼ 6 quasars. At the wavelength of Lyα at z ∼ 10.6, we identify three other emission line candidates within the integral field unit field of view with no UV rest-frame counterpart visible in deep images from the JWST/NIRCam. If confirmed, this could be the first evidence that the local region of GN-z11 represents a candidate protocluster core, forming just 400 Myr after the Big Bang. We give a first estimate of the dark matter halo mass of this structure $ \left(M_{\mathrm{h}} =2.96 ^{+0.44}_{-0.39} \times 10^{10}\,M_{\odot}\right) $, which is consistent with a Coma-like cluster progenitor.