Cosmological simulations

Until a few years ago, studies of galaxy formation have been affected by uncertainties both in the underlying cosmology as well as in the most relevant physical processes. Now we are in the so called "precision cosmology" era, which means that the background model is relatively well defined by a wealth of observations, in particular those of fluctuations in the cosmic micro-wave background. So, we can compute with reasonable confidence the evolution of the dynamically dominant dark matter (DM) component, ruled by gravity. Starting from a reasonable spectrum of primordial density fluctuations, over-density regions above the linear regime collapse into sheets and filaments. Then, matter mainly flows along filaments into dark matter halos. These halos merge to form bigger and bigger halos (hierarchical clustering).The general outcomes of these gravity-only simulations confirm and deepen those obtained by means of analytical analysis, yielding a broad outline of the formation of cosmic structures: galaxies and clusters. Indeed, a full understanding of the processes leading to the formation of cosmic structures, galaxies in particular, would require the much more demanding task of treating the complex physic of luminous (baryonic) matter. Galaxy formation, which occurs in DM halos, involves a complex web of processes: merging of dark matter halos, cooling of gas, collapse and star formation from cold gas, energy input into gas from SNae explosions and winds (energetic feedback), chemical enrichment of gas and stars (chemical feedback), galaxy mergers, luminosity evolution of stellar populations, absorption of starlight by dust and re-emission in IR+sub-mm, formation of super massive black holes, the ensuing AGN activity and its feedback on the interstellar medium. To follow from first principles all these processes in a fully cosmological context, it would span a dynamical range from << 1pc to >10 Mpc. Moreover, many of processes above are still poorly understood. Thus, simulations genuinely from first principles are at present impossible. Two complementary approaches are usually followed: (i) numerical simulations including gas, i.e. smooth particle hydro-dynamical (SPH) simulations, accounting for phenomenological prescriptions of sub-grid physics (e.g. star formation, feedback, SMBH growth), (ii) semi-analytical models (SAMs), using the prescription approach for every process involving baryons. At the OAPd we are deeply involved in both these projects. In particular, SPH simulations focusing on the bar growth and its evolution, for the first time in a fully cosmological frame , and the first SAM model accounting for the feedback from star formation and AGN activity, have been recently performed. Results from both these approaches help us to shed light on, and may be to solve, several crucial points concerning galaxy evolution.

Cosmological Simulations - PAST STUDIES

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