The relation of galaxies and dark matter haloes to the filamentary cosmic web

ABSTRACT We use the Millennium Simulation to study the relation of galaxies and dark matter haloes to the cosmic web. We define the web as the unique, fully connected, percolating object with (unsmoothed) matter density everywhere exceeding 5.25 times the cosmic mean. This object contains 35 per cent of all cosmic mass but occupies only 0.62 per cent of all cosmic volume. It contains 26 per cent of dark matter haloes of mass $10^{11} \, {\rm M}_\odot$, rising to 50 per cent at $10^{12.7} \, {\rm M}_\odot$, and to ${>}90 {{\, \rm per\, cent}}$ above $10^{14} \, {\rm M}_\odot$. In contrast, it contains 45 per cent of all galaxies of stellar mass $10^{8.5} \, {\rm M_\odot}$, rising to 50 per cent at $10^{10} \, {\rm M}_\odot$, to 60 per cent at $10^{11} \, {\rm M}_\odot$, and to 90 per cent at $10^{11.5} \, {\rm M}_\odot$. This difference arises because a large fraction of all satellite and backsplash galaxies are part of the cosmic web. Indeed, more than 50 per cent of web galaxies are satellites for stellar masses below that of the Milky Way, rising to about 70 per cent below $10^{10} \, {\rm M}_\odot$, whereas centrals substantially outnumber satellites in the non-web population at all stellar masses. As a result, web galaxies have systematically lower specific star formation rates (sSFRs) than non-web galaxies. For the latter, the distributions of stellar mass and sSFR are almost independent of web distance. Furthermore, for both central and satellite galaxies, the sSFR distributions at a given stellar mass are very similar in and outside the web, once differences in backsplash fraction are accounted for. For the galaxy formation model considered here, differences between web and non-web galaxy populations are almost entirely due to the difference in halo mass distribution between the two environments.