as well as radiative transfer calculations to realistically propagate the photons from local galaxies into their surrounding environment [@art:eagle; @art:eagle2]. We make use of the largest box that is available in the public data release [@art:eagle_pub2]. The box contains $1504^3$ dark matter particles with masses of $6.57\times10^6 h^{-1}\mathrm{M_\odot}$ and initially the same amount of baryonic particles with masses of $1.23\times10^6\, h^{-1}\mathrm{M_\odot}$. The volume of the simulation is $100\,\mathrm{Mpc}^3$.\
The dataset provides us with the smooth particle hydrodynamical (SPH) particle data, namely the gas density, gas temperature and smoothing length of the particles, as well as the positions of halos. We calculate the HI properties of the gas, including the 21 cm differential brightness temperature signal directly on the SPH particles. In order to estimate the hydrogen neutral fraction in each cell, we assume ionization-recombination equilibrium and use the temperature directly from the simulation. The ionization fraction additionally depends on the electron density and the strength of the photo-ionizing background. We note that in this simulation, for starforming particles the temperature represents a parameterization of the effective pressure of the multiphase interstellar medium. Their temperature should therefore not be used to compute the ionization fraction. Since these particles will mostly be 