The image shows a slice of a numerical model for dark matter (grey points) and galaxies. Two types of galaxies are shown: bright emission line galaxies (filled circles) and those galaxies hosted by gravitational potentials above a threshold such that the number density equals that of the emission line galaxies (open circles). This figure illustrates that emission line galaxies trace different features from the cosmic web than galaxies selected by either their stellar mass or the strength of the gravitational potential where they have formed.