Seeking Observable Imprints of Small-Scale Structure on the Properties of Dark Matter Haloes

Abstract

The characteristic prediction of the Cold Dark Matter (CDM) model of cosmological structure formation is that the Universe should contain a wealth of small-scale structure -- low-mass dark matter haloes and subhaloes. However, galaxy formation is inefficient in their shallow potential wells and so we expect these low-mass haloes and subhaloes to be dark. Can we tell the difference between a Universe in which low-mass haloes are present but dark and one in which they never formed, thereby providing a robust test of the CDM model? We address this question using cosmological simulations to examine how properties of low-mass haloes that are potentially accessible to observation, such as their spatial clustering, rate of accretions and mergers onto massive galaxies and the angular momentum content of massive galaxies, differ between a LCDM model and dark matter models in which low-mass halo formation is suppressed. Adopting an effective cut-off mass scale M_cut below which small-scale power is suppressed in the initial conditions, we study dark matter models in which M_cut varies between 5e9 Msol/h and 1e11 Msol/h, equivalent to the host haloes of dwarf and low mass galaxies. Our results show that both the clustering strength of low-mass haloes around galaxy-mass primaries and the rate at which they merge with these primaries is sensitive to the assumed value of M_cut; in contrast, suppressing low-mass halo formation has little influence on the angular momentum content of galaxy-mass haloes -- it is the quiescence or violence of a halo's assembly history that has a more marked effect. However, we expect that measuring the effect on spatial clustering or the merger rate is likely to be observationally difficult for realistic values of M_cut, and so isolating the effect of this small-scale structure would appear to be remarkably difficult to detect, at least in the present day Universe.