2022A&A...667L...3L

em Context. According to the current paradigm of galaxy formation, the first galaxies likely formed within large dark matter haloes. The fragmentation of these massive haloes led to the formation of galaxy protoclusters, which are usually composed of one to a few bright objects, surrounded by numerous fainter (and less massive) galaxies. These early structures could have played a major role in reionising the neutral hydrogen within the first billion years of the Universe, especially if their number density is significant.

em Aims. Taking advantage of the unprecedented sensitivity reached by the James Webb Space Telescope (JWST), galaxy protoclusters can now be identified and studied in increasing numbers beyond z ≥ 6. Characterising their contribution to the UV photon budget could supply new insights into the reionisation process. Methods. We analysed the first JWST dataset behind SMACS0723-7327 to search for protoclusters at z ≥ 6, combining the available spectroscopic and photometric data. We then compared our findings with semi-analytical models and simulations.

em Results. In addition to two bright galaxies (≤26.5 AB in F277W), separated by ∼11″and spectroscopically confirmed at z_spec = 7.66, we identify six additional galaxies with similar colours within a θ ∼ 20″ radius (corresponding to R ∼ 60 - 90 kpc in the source plane). Using several methods, we estimate the mass of the dark matter halo of this protocluster as ∼3.3 × 10¹¹ M_⊙, accounting for magnification, consistent with various predictions. The physical properties of all protocluster members are also in excellent agreement with what has been previously found at lower redshifts: star formation main sequence and protocluster size. This detection adds to just a few protoclusters currently known in the first billion years of the universe. These z ≥ 7 galaxy protoclusters may play an important role in cosmic reionisation.