A projected slice through a cosmological volume at redshift |$z=5.5$|⁠. We average the values over a thickness of 10 per cent of the box size and show the neutral hydrogen fraction, temperature, and gas density. The morphology is dominated by a filament containing high-density, neutral gas across which stars and galaxies are forming. Feedback leads to the heating of the medium around the filament. The IGM contains a smooth UV radiation field and is almost isothermal, with a temperature between 10 000  and 20 000 K. For this plot, we used the base simulation with RT on the GPU.