THE GROWTH OF DARK MATTER HALOS: EVIDENCE FOR SIGNIFICANT SMOOTH ACCRETION

We study the growth of dark matter halos in the concordance ΛCDM cosmology using several N-body simulations of large cosmological volumes. We construct merger trees from the Millennium and Millennium-II Simulations, covering the ranges 109–1015 M☉ in halo mass and 1–105 in merger mass ratio. Our algorithm takes special care of halo fragmentation and ensures that the mass contribution of each merger to halo growth is only counted once. This way the integrated merger rate converges and we can consistently determine the contribution of mergers of different mass ratios to halo growth. We find that all resolved mergers, up to mass ratios of 105:1, contribute only ≈60% of the total halo mass growth, while major mergers are subdominant, e.g., mergers with mass ratios smaller than 3:1 (10:1) contribute only ≈20% (≈30%). This is verified with an analysis of two additional simulation boxes, where we follow all particles individually throughout cosmic time. Our results are also robust against using several halo definitions. Under the assumption that the power-law behavior of the merger rate at large mass ratios can be extrapolated to arbitrarily large mass ratios, it is found that, independent of halo mass, ≈40% of the mass in halos comes from genuinely smooth accretion of dark matter that was never bound in smaller halos. We discuss possible implications of our findings for galaxy formation. One implication, assuming as is standard that the pristine intergalactic medium is heated and photoionized by UV photons, is that all halos accrete >40% of their baryons in smooth “cold” T ≳ 104 K gas, rather than as warm, enriched, or clumpy gas or as stars.