Toward a Universal Model for the Mass Accretion History of Dark Matter Halos in Cosmological Simulations

It is now well established that a large percentage of the energy density of the universe is in the form of non-baryonic dark matter, a still unidentified type of matter that does not emit or interact with electromagnetic radiation. At the present time, most of the dark matter is virialized in large structures called ‘halos’ which formed via hierarchical clustering, a series of subsequent mergers of smaller halos originating from the growth of the perturbations of the density field of the early universe. The study of this tree of mergers, and of its main branch, is of primary importance in understanding the properties of halos at the present time. The primary tools for the study of the evolution of structures in the non-linear regime are large numerical simulations, that evolve some suitable initial conditions by numerical integration of the gravity equations. We will present our set of simulations, partly developed in the context of this work. By exploiting the large statistic and dynamical range provided, we will present our refinement and expansion of a previous model for the mass accretion history of the halos, greatly expanding its applicability. In particular, our model will allow us to characterize both the median mass accretion history as well as the full halo-to-halo distribution, and we will discuss some applications. Studying the scatter of the distribution, we will present a preliminary analysis of the percentile distributions of mass accretion histories. Despite non-conclusive results, we will provide a characterization that can be useful in checking the validity of methods to generate synthetic merger trees. We will argue for the universality of the model, which allows us to apply our results to massive neutrino cosmologies. Multiple experiments in recent years confirmed the existence of flavor oscillations in the propagation of neutrino fluxes, a phenomenon usually interpreted as the effect of a nonzero mass for the neutrinos together with a mixing of the flavor and mass eigenstates. The presence of non-zero neutrino masses has severe cosmological implications, causing in particular a slowdown in the growth and evolution of the structures on small scales. We will illustrate how to modify our model to account for these effects.