Abstract

JWST has transformed our view of the early Universe, revealing complex structures and active supermassive black holes much earlier than previously thought. I will present recent results from JWST programs that probe galaxy overdensities and protoclusters at z=4–10 through deep spectroscopy and multi-wavelength imaging, complemented by X-ray observations. These results show that environmental effects, mergers, and feedback were already shaping galaxy growth and AGN activity within the first billion years. Highlights include the discovery of a dual AGN system at z ∼ 5.4 embedded in a Lyα-emitting nebula within a forming galaxy cluster, and the detection of diffuse X-ray emission in a massive protocluster at z ∼ 5.7, offering the first direct evidence of early virialisation at cosmic dawn. To place these results in a broader context, I will also present complementary studies of dusty quasar environments and large-scale structures traced by submillimetre and radio observations, as well as investigations of extended Lyα nebulae that probe the multi-phase circumgalactic medium in dense regions. Taken together, these results suggest that the seeds of today’s massive clusters were already taking shape at cosmic dawn, and offer a more unified picture of how galaxies, black holes, and gas co-evolve in the most massive environments.