Projection of the gas density and mock observations. Panels on the left are images of a massive galaxy at different epochs and the panels on the right are for a less massive galaxy. First row: gas density projections a of 1.4 Mpc at different epochs, while the second and fourth rows are zoomed-in gas density projections with face-on and edge-on views of the galaxy, respectively. Third and fifth rows are SKIRT mock observations in face-on and edge-on direction. Stellar mass and halo mass of each galaxy at each epoch are given in the second row in log scale. For a galaxy at a given epoch, the second to fifth panels are on the same scale and the white bar in third row indicates 5 kpc. Gas density maps share the same colour scheme as given in the colour bar.