Hubble parameter, $T_{\rm CMB}$ the temperature of the Cosmic Microwave Background (CMB), $T_{\rm s}$ the spin temperature of the gas and $dv_r/dr$ is the comoving gradient of the comoving velocity along the line of sight. The signal can be inferred from the gas density (including clumping) and temperature assuming an ionizing background radiation model. In , these quantities were determined from the density field of a dark matter (DM) only simulation by assuming that the baryonic matter follows the DM density field perfectly. From there, the ionization and neutral fractions and the temperature were derived in thermal and ionization equilibrium. The HI emission in that case is completely governed by the UVB, the density field and the cosmology.\
However, we know from observations that filaments contain galaxies and quasars that provide an extra local source of heating and ionization. This, together with shock-heating negatively impacts the HI 21 cm signal. Therefore, for this study, we adopt a more sophisticated hydrodynamical simulation that includes both these effects, resulting in more realistic conditions in the IGM. This will allow us to better estimate the strength of the filament signal. We furthermore apply a strategy to find filaments that is applicable to both 