we chose to use the Bisous model code by @art:tempel14 [@art:tempel16] to find filaments in the EAGLE simulation. It has also been widely applied to observations. The Bisous algorithm models the three-dimensional structures in the distribution of the galaxies through a marked point process and only requires the galaxy positions as input. The statistical nature of the inferred filaments means that there is a significant probability that some of the identified filaments are not real, since a group of galaxies that formed along a line by chance, but whose underlying density field is not connected by a filament could still be inferred by the code as a connected filament. To minimize this effect in our sample, we only consider filaments that are longer than 5 $h^{-1}$Mpc. Moreover, the spatial distribution of the filaments should be closely connected to large galaxies. Therefore, we limit the sample to galaxies with masses above $10^8M_{\odot}$ in order to trace the stronger filaments, as well as those having non-zero starformation rate in EAGLE since filament galaxies can accrete gas from the filament and can thus be expected to have some ongoing starformation. The mean brightness temperature of HI 21 cm emission in a slice of 