RIOJA I. The core of the highest redshift galaxy overdensity at z=7.88 confirmed by NIRSpec/JWST | Projected gas density (panel a) and [O iii] 5008 °A distribution (panel b) for a galaxy sample FL957 (Ceverino et al. 2017) at z = 7.7. Each panel shows a region with a side length and depth of 50 kpc and 10.6 kpc, the latter corresponds to the NIRSpec IFS field of 3'' with magnification of µ ~ 2. Panels (c) and (d) show the following evolution of FL957 at z = 7.5 and 7.3, demonstrating that the system finally becomes a large massive galaxy by z = 7.3
