Large-scale structure in a universe with mixed hot and cold dark matter

Abstract

A UNIVERSE whose density is dominated by cold dark matter (CDM) has been considered the standard model for large-scale structure formation¹, but it has had difficulty in matching the relatively quiet velocity field of galaxies² and the observed structure on very large scales³. By contrast, models with a mixture of COM and hot dark matter (HDM) have more power on large scales^4–9, and seem more able to fit the excess large-scale power seen in galaxy surveys³ and the microwave background fluctuations recently measured by COBE^10,11. Using high-resolution numerical simulations, we examine the formation of structure in a mixed dark matter model containing 70% COM and 30% HDM, the latter in the form of massive neutrinos. This model behaves like a CDM model in which the biasing factor (the relative magnitude of structure in the dark and visible components) varies from 2.5 on small scales to <1 on large scales, and can provide a consistent explanation of both the shape of the observed fluctuation spectrum and the difference in estimates of the cosmic density, Ω, on small and large scales.